Controlled growth of a nanostructure on a substrate

ABSTRACT

The present invention provides for nanostructures grown on a conducting substrate, and a method of making the same. The nanostructures grown according to the claimed method are suitable for manufacturing electronic devices such as an electron beam writer, and a field emission display.

CLAIM OF PRIORITY

This application claims the benefit of priority of Swedish provisionalapplication serial no. 0500926-1, filed Apr. 25, 2005, and of Swedishprovisional application serial no. 0501888-2, filed Aug. 25, 2005, andto U.S. provisional application Ser. No. 60/772,449, filed Feb. 10,2006, all of which are incorporated herein by reference in theirentirety.

FIELD OF THE INVENTION

The present invention generally relates to nanostructures and methodsfor their growth. The present invention more particularly relates tomethods of controlling the growth of nanostructures such as carbonnanofibers which enables manufacture of electron emission based devicessuch as electron beam writers and field emission displays.

BACKGROUND

Relentless efforts at miniaturization are bringing traditional CMOSdevices to the limit where the device characteristics are governed byquantum phenomena; in such regimes, perfect control is impossible toachieve. This has engendered a need for finding alternative newmaterials to fabricate devices that will possess at least the same oreven better performance than existing CMOS devices but with greatercontrol.

The miniaturization of CMOS devices has hitherto been governed by atrend—often called Moore's law—in which electronic components shrink insize by half every 30 months. The International Technology Roadmap forSemiconductors (ITRS) has established a projected growth curve accordingto this model. The demands for speed, high integration level, highperformance and low production costs attendant on such a rate ofprogress are very stringent. Consequently, the problems associated withthe physical and electrical characteristics of traditional materialsused for making devices have escalated. Hence there is a need to searchfor alternative solutions to the problems that will ultimately impedethe progress of silicon technology in the immediate future. This meansthat devising innovative material and process solutions is critical tosustaining the projected rate of growth.

The choice of new materials is however limited by factors such ascompatibility with existing production methods, reproducibility ofmanufacture and cost. Some problems that existing technology materialshave faced are as follows.

High power consumption due to leakage current: currently, the deviceperformance is degraded due to high leakage current through gate oxide(which is very thin). This in turn increases the leakage current in theoff state, and hence increases power consumption, which in turn reducesthe life time of a battery.

Poor performance of Cu interconnects: due to its low resistivity, copperis used for making interconnects that are used for connecting variouscomponents to one another, as well as devices and circuits with theoutside world. Due to the dramatic reduction in the size of thecomponents, interconnects based on copper material are now showing poorperformance in terms of current carrying capacity and lifetime of thewires. This in turn reduces the lifetime of a processor. No solutioncurrently exists for interconnects that will efficiently connect thedevices in a circuit with those outside of the circuit, in time to meetthe projected demand for current density over the next several years.

Demand for high aspect ratio structures: today the aspect ratio ofcontact holes for interconnects in DRAM staked capacitors has reached12:1 and is expected to increase to 23:1 by the year 2016. Creating suchhigh aspect ratio contacts with straight walls poses substantialtechnological challenges, not least because void-free filling withmetals (also known as vias) of such high aspect ratio features isextremely difficult.

High heat dissipation: modern microprocessors generate inordinateamounts of heat. Heat dissipation has been increasing steadily as thetransistor count and clock frequency of computer processors hasincreased. In particular, for example, copper interconnects of the sizesrequired for current and future devices generate so much heat that theirelectrical resistance is increased, thereby leading to a decreasedcapacity to carry current. However a practical solution for cooling ofsuch systems which will not eventually exceed the power budget forprocessors has yet to be found.

In short, for all these reasons, it has become necessary to search foralternative materials and processing technology.

Carbon nanostructures, including carbon nanotubes (CNTs) and nanofibers,are considered to be some of the most promising candidates for futuredevelopments in nano-electronics, nano-electromechanical systems (NEMS),sensors, contact electrodes, nanophotonics, and nano-biotechnology. Thisis due principally to their one dimensional nature, and their uniqueelectrical, optical and mechanical properties. In contrast to thefullerenes, such as C₆₀ and C₇₀, whose principal chemistry is based onattaching specific functionalities thereby giving rise to specificproperties, CNTs offer an almost limitless amount of variation throughdesign and manufacture of tubes of different diameters, pitches, andlengths. Furthermore, whereas the fullerenes offer the possibility ofmaking a variety of discrete molecules with numerous specificproperties, carbon nanotubes provide the possibility to makemolecular-scale components that have excellent electrical and thermalconductivity, and strength. (See, e.g., Nanoelectronics and InformationTechnology, R. Waser (Ed.), Wiley-VCH, 2003, at chapter 19.)

Carbon nanotubes and carbon nanofibers have been considered for bothactive devices and as interconnect technology at least because theirelectrical and thermal properties and their strength. For example, thehigh electron mobility of carbon nanotubes (79,000 cm²/Vs) surpassesthat of state-of-the-art MOSFET devices (see, e.g., Durkop, T., et al.,Nano Letters, 4(1), 35, (2004)). Furthermore, the extremely high currentcarrying capacity of carbon nanotubes (10¹⁰ A/cm²) (see, e.g., Wei, B.Q., et al., Appl. Phys. Lett., 79(8), 1172, (2001)), when compared withcopper interconnects (˜10⁶ A/cm²), means that carbon nanotubespotentially possess the solution to the severe problems forinterconnects projected in ITRS.

The anisotropic thermal conductivity of nanotubes/nanofibers (6,000W/Km) (see, e.g., Hoenlien, W., et al., IEEE Trans. Compon. andPackaging Tech., 27(4), 629, (2004)) is also exceptionally promising forsolving problems of heat dissipation.

Finally, the high E-modulus (representing the strength of a material) ofindividual nanotubes (as high as 1 TPa) has made them a good choice forboth composite materials and for nanoelectromechanical devices.

In general, it is highly desirable to fabricate electronic devices thatare compatible with existing complementary metal oxide semiconductor(CMOS) fabrication techniques. A prerequisite for exploring CNTs in anindustrial process is to be able to control mass production of deviceswith high reproducibility. Due to high purity and high yield, chemicalvapor deposition (CVD) is a popular and advantageous growth method thatoffers the potential to grow nanotubes at an exact location with controlover their length, diameter, shape and orientation.

Hence for many electronic, nanoelectromechanical systems andinterconnect applications the integration possibilities of carbonnanostructures into existing CMOS-based electronic industrialmanufacturing processes is expected to be a ground breakingtechnological breakthrough. However, there are many engineering andmaterials issues inherent to CMOS-compatible device fabricationprocesses that need to be addressed before such integration can takeplace. Solutions to these issues have so far been long-awaited.

For instance, there are problems related to growth of nanostructures.Although numerous techniques have been developed and demonstrated toproduce carbon based nanostructures, all possess drawbacks regardingmass production and integration into existing industry manufacturingprocesses. Prominent drawbacks are: (a) control over predictablemorphology with either semiconducting or metallic properties, (b)precise localization of the grown individual structures, and (c)predictable electrical properties at the interface between the grownnanostructures and the substrate. There is no known single solution tosolve all the aforementioned drawbacks. The most prominent techniquesfor synthesizing carbon nanostructures include arc discharge (see, e.g.,Iijima, S., Nature, 354, 56, (1991); and Kratschmer, W.; Lamb, L. D.;Fostiropoulos, K.; Huffman, D. R., Nature, 347, 354, (1990)), laservaporization (see, e.g., Kroto, H. W.; Heath, J. R.; O'Brien, S. C.;Curl, R. F.; Smalley, R. E. Nature, 318, 162, (1985)), catalyticchemical-vapor deposition (CCVD), also referred to as CVD, (Cassell, A.M.; Raymakers, J. A.; Jing, K.; Hongjie, D., J. Phys. Chem. B, 103,(31), (1999)), and catalytic plasma enhanced chemical-vapor deposition(C-PECVD) (Cassell, A. M.; Qi, Y.; Cruden, B. A.; Jun, L.; Sarrazin, P.C.; Hou Tee, N.; Jie, H.; Meyyappan, M., Nanotechnology, 15(1), 9,(2004); and Meyyappan, M.; Delzeit, L.; Cassell, A.; Hash, D., PlasmaSources, Science and Technology, 12(2), 205, (2003)), all of whichreferences are incorporated herein by reference in their entirety. Dueto high purity and high yield, chemical vapor deposition (CVD) is apopular and advantageous growth method, and indeed, among all of theknown growth techniques, CMOS compatibility has been demonstrated onlyfor the CCVD method. (See, Tseng, et al. (Tseng, Y.-C.; Xuan, P.; Javey,A.; Malloy, R.; Wang, Q.; Bokor, J.; Dai, H. Nano Lett. 4(1), 123-127,(2004), incorporated herein by reference) where a monolithic integrationof nanotube devices was performed on n-channel semiconductor (NMOS)circuitry.)

There are specific problems related to control of the properties ofgrown materials. Even though numerous different alternative growthmethods exist for growing carbon nanostructures, controlling theinterface properties between the nanostructures and the substrates, thebody of the nanostructures, and the tip of the nanostructures are notyet demonstrated to be well controlled by utilizing a single method ofgrowth.

CVD typically employs a metal catalyst to facilitate carbonnanostructure growth. The main roles of the catalyst are to break bondsin the carbon carrying species and to absorb carbon at its surface andto reform graphitic planes through diffusion of carbon through or aroundan interface (see, e.g., Kim, M. S.; Rodriguez, N. M.; Baker, R. T. K.,Journal of Catalysis, 131, (1), 60, (1991); and Melechko, A. V.;Merkulov, V. I.; McKnight, T. E.; Guillorn, M. A.; Klein, K. L.;Lowndes, D. H.; Simpson, M. L., J. App. Phys., 97(4), 41301, (2005),incorporated herein by reference).

However, the growth of nanotubes is usually carried out on silicon orother semiconducting substrates. Growth from such metal catalysts onconducting metal substrates or metal underlayers is almost lacking. Thisis because it has been found that it is hard to make a good contactbetween a growing nanostructure and a conducting substrate with goodquality grown nanostructures in terms of control over diameter, lengthand morphology. Nevertheless, for making CMOS-compatible structures, itis necessary to use a conducting substrate. In particular, this isbecause a metal substrate, or base layer, acts as bottom electrode forelectrical connection to the nanostructures.

Nevertheless, growth of nanostructures on CMOS compatible conductingsubstrates has proved to be far from trivial, at least because differentmetals require different conditions, and also because it has provendifficult to control the properties of the nanostructures grown on suchsubstrates with predictable control over diameter, length and morphologyof the grown structures and with predictable interface propertiesbetween the nanostructures and the substrate.

A method for producing arrays of carbon nanotubes on a metal underlayer,with a silicon buffer layer between the metal underlayer and a catalystlayer, is described in U.S. Patent Application Publication No.2004/0101468 by Liu et al. According to Liu, the buffer layer preventscatalyst from diffusing into the substrate and also prevents the metalunderlayer from reacting with carbon source gas to, undesirably, formamorphous carbon instead of carbon nanostructures. In Liu, the processinvolves, inconveniently, annealing the substrate in air for 10 hours at300-400° C. to form catalyst particles, via oxidation of the catalystlayer, prior to forming the nanostructures. Each catalyst particle actsas a seed to promote growth of a nanostructure. The method of Liu,however, does not permit control of the composition or properties of thenanostructures and the nanotubes produced are curved and disorganized.

An additional goal is fabrication of carbon based nano-electromechanical structures (NEMS). Extensive theoretical analysis ontwo-terminal and three-terminal carbon based NEMS (C-NEMS) structureswere performed by Dequesnes et al. (Dequesnes, M.; Rotkin, S. V.; Aluru,N. R., Nanotechnology, 13(1), 120, (2002)) and Kinaret et al. (Kinaret,J. M.; Nord, T.; Viefers, S., Applied Physics Letters, 82(8), 1287,(2003)) respectively, all of which references are incorporated herein byreference in their entirety. The model developed by Kinaret et al. forthree-terminal NEMS device consists of a conducting carbon nanotube(CNT) placed on a terraced Si substrate and connected to a fixed sourceelectrode which they have called it “nanorelay.” Recently Lee et al.(Lee, S. W. L., et al., Nano Letters, 4(10), 2027, (2004), incorporatedherein by reference) have demonstrated the characteristics of such threeterminal nanorelay structures experimentally. However, the experimentalapproach by Lee et al. for fabricating such devices is time consumingand the technology is heavily dependent on the sonicated CNF solutionswhich usually do not possess any control over the length and thediameter of the CNF: the functional part of the device. Therefore, it isdesirable to develop a technology for fabricating such structures withpredictable behavior.

Accordingly, there is a need for a method of growing carbonnanostructures on a metal substrate in such a way that variousproperties of the nanostructures can be controlled.

The discussion of the background to the invention herein is included toexplain the context of the invention. This is not to be taken as anadmission that any of the material referred to was published, known, orpart of the common general knowledge as at the priority date of any ofthe claims.

Throughout the description and claims of the specification the word“comprise” and variations thereof, such as “comprising” and “comprises”,is not intended to exclude other additives, components, integers orsteps.

SUMMARY OF THE INVENTION

A nanostructure assembly comprising: a conducting substrate; ananostructure supported by the conducting substrate; and a plurality ofintermediate layers between the conducting substrate and thenanostructure, the plurality of intermediate layers including at leastone layer that affects a morphology of the nanostructure and at leastone layer to affect an electrical property of an interface between theconducting substrate and the nanostructure.

A multilayer interface between a catalyst and a substrate having: atleast one layer to control morphology, and at least one layer to controlan electrical interface between a nanostructure and base layer. In themultilayer interface, at least one layer is preferably of asemiconducting material such as silicon or germanium.

A nanostructure supported upon a metal substrate, wherein metal isinterdiffused with a semiconducting layer between the nanostructure andthe substrate.

The present invention also contemplates forming nanostructures at hightemperatures but without prior annealing of a catalyst layer on whichthe nanostructures are grown. Preferably the temperatures employed areless than 750° C.

The present invention also contemplates the formation of nanostructuresformed not of carbon but of other solid state materials such as GaN,GaAs, InP, InGaN, ZnO, Si. In general, semiconducting nanostructures arebased on a combination such as II-VI or III-V materials from theperiodic table of the elements. Examples of appropriate conditions formaking such nanostructures are further described herein.

The present invention also contemplates a “lift-off” method offabricating individual fibers: lift-off of polymer layer to provideindividual layers.

Nanostructures formed according to the present invention may be used asinterconnects, current carrying conductors, anisotropic heat directingmedia, can be integrated into components: active/passive devices likediodes, transistors, capacitors, inductors, field emitting devices,optical devices, X-ray emitting devices, sensors, electrochemical probesetc.

A precursor for a nanostructure assembly, comprising: a conductingsubstrate; a catalyst layer; and a plurality of intermediate layersbetween the conducting substrate and the catalyst layer, the pluralityof intermediate layers including at least one layer to affect morphologyof a nanostructure to be formed on the catalyst layer and at least onelayer to affect electrical properties of an interface between thesupport layer and the nanostructure.

By having a layer of material between the catalyst and the substrate, itis possible to influence the texture of the final catalytic particlesand hence influence the growth mechanism and morphology of the grownnanostructures.

A carbon nanostructure assembly comprising: a metal layer; a carbonnanostructure; and at least one intermediate layer between the metallayer and the carbon nanostructure, the at least one intermediate layerincluding a semiconductor material, a catalyst, and a metal from themetal layer.

A carbon nanostructure assembly comprising: a conducting substrate; alayer of amorphous silicon on the conducting substrate; and a layer ofcatalyst on the layer of amorphous silicon, wherein the carbonnanostructure is disposed on the catalyst.

A carbon nanostructure, comprising: a substantially straight generallycylindrical carbon nanostructure having a conical angle less than 2degrees.

An array of carbon nanostructures supported on a substrate, wherein eachcarbon nanostructure in the array comprises: a conducting substrate; aplurality of intermediate layers on the conducting substrate; a catalystlayer on the intermediate layers; and a carbon nanostructure on thecatalyst layer, wherein said each carbon nanostructure is spaced apartfrom any other carbon nanostructure in the array by between 70 nm and200 nm.

A method of forming a nanostructure, comprising: depositing a layer ofsemiconducting material on a conducting substrate; depositing a catalystlayer on the semiconducting layer; without first annealing thesubstrate, causing the substrate to be heated to a temperature at whichthe nanostructure can form; and growing a nanostructure on the catalystlayer at the temperature.

A method of forming a nanostructure precursor, comprising: depositing asacrificial layer on a conducting substrate; forming a plurality ofapertures in the sacrificial layer; depositing an intermediate layer ofsemiconducting material over the sacrificial layer and on the substratein the apertures; depositing a catalyst layer over the intermediatelayer; and lifting off the sacrificial layer to leave portions of theintermediate layer and catalyst layer corresponding to the apertures onthe substrate.

An electron beam writer, comprising: a support; an insulating layer onthe support; a third layer of material on the insulating layer, arrangedto form a cavity; a metal electrode on the insulating layer, in thecavity; a nanostructure built upon the metal electrode; and an electrodelayer deposited on the third layer of material.

An electron beam writer, comprising: a nanostructure having a base and atip, wherein the base is affixed to a first electrode; a plurality ofsecond electrodes disposed around the nanostructure; and electricalcircuitry that connects the first electrode to the plurality of secondelectrodes, and is configured to: cause a voltage difference to arisebetween the first electrode and the plurality of second electrodes;cause electrons to be emitted from the tip; and cause the tip to move inspace towards one of the plurality of second electrodes.

A field emission device, comprising: a plurality of pixels, wherein eachpixel comprises: a conducting substrate; a plurality of nanostructuresdeposited on the conducting substrate, wherein a plurality ofintermediate layers between the nanostructures and the conductingsubstrate includes at least one layer of semiconducting material; andwherein the conducting substrate forms an electrode that is inelectrical communication with a voltage source and second electrode; andwherein the second electrode has a coating of phosphor; and wherein uponapplication of a voltage between the conducting substrate and the secondelectrode, the nanostructures emit electrons towards the phosphorcoating.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of a carbon nanofiber.

FIG. 2 shows a flow-chart of an overall process according to the presentinvention.

FIGS. 3A and 3B show various configurations of the present invention.

FIG. 4 shows a multilayer stack between a metal layer and ananostructure, and having various segments of different functionalities.

FIG. 5 shows a step in creation of an individual nanostructure.

FIG. 6 shows an individual nanostructure with a single layer between thenanostructure body and a metal substrate.

FIG. 7 shows an individual nanostructure.

FIG. 8 shows an individual nanostructure having a multilayer stack.

FIG. 9 shows an embodiment of a nanostructure.

FIG. 10 shows an intermediate stage in a process of making ananostructure.

FIG. 11 shows an example of growth of a nanostructure.

FIG. 12 shows layers that control properties of an individualnanostructure.

FIG. 13 shows an individual nanostructure as part of an electriccircuit.

FIG. 14 shows an electric circuit configured to use a carbonnanostructure.

FIG. 15 shows an individual nanostructure as part of an electric device.

FIG. 16 shows an individual nanostructure as an electric/optical device.

FIG. 17 shows an individual nanostructure as part of a Schottky Barrier.

FIG. 18 shows an individual nanostructure as part of a Schottky Barrier.

FIG. 19 shows an energy level diagram for an interface in a structureaccording to FIG. 18.

FIGS. 20A-20C show various views of a field emission device usingnanostructures according to the present invention.

FIGS. 21A-21C show various embodiments of an electron-beam emitter usingnanostructures according to the present invention.

FIGS. 22A-22D show various embodiments of electrode configurations in anelectron-beam writer using nanostructures according to the presentinvention.

FIG. 23 shows an electron-beam writer configured to write on asubstrate.

FIG. 24 shows a horizontal configuration of an electron-beam writer.

FIG. 25 shows an exemplary electron-beam writer using a nanostructure.

FIG. 26A is a transmission electron microscopy (TEM) micrograph of acarbon nanofiber grown on a tungsten underlayer. FIG. 26B shows: (a) TEMmicrograph of a nanofiber grown on a W metal underlayer; (b) acorresponding EDS spectrum taken at the tip of the fibers (catalystregion); and (c) an EDS spectrum taken at the base of the fibers(underlayer region).

FIGS. 27A & B show schematics of a layers on a conducting underlayer ona support, with Si as intermediate layer (FIG. 27A), and Ni catalystdeposited directly on the metal underlayer (FIG. 27B).

FIG. 28: SEM micrographs of metal underlayers after growth sequence.Only W and Mo metal underlayers facilitated appreciable CNT growth. Inthis set of experiments Ni was evaporated directly on the metalunderlayers. Standard growth conditions (V_(B)=−400 V, C₂H₂:NH₃=1:5,time=15 min., T=700° C.) were used for all cases. All scale bars are 1μm except FIG. 27( c).

FIG. 29. Density of individual nanostructures μm⁻² area for the case ofMo and W metal underlayers without amorphous Si layer.

FIG. 30. SEM micrograph of the samples after 15 min. of CVD growth. Thepresence of Si facilitated the growth of nanotubes on some metalunderlayers which was not possible in the previous set of experiments.Standard growth conditions (V_(B)=−400 V, C₂H₂:NH₃=1:5, time=15 min,T=700° C.) were used for all cases. All scale bars are 1 μm.

FIG. 31. Particle size distribution for four most promising metalunderlayer samples: (a) platinum; (b) palladium; (c) tungsten; (d)molybdenum. The nanotube diameter distribution was plotted averagingthree different images as shown in FIG. 29 for each metal underlayer.

FIG. 32. Top-view SEM images of CNTs grown on (a) platinum; (b)palladium; (c) tungsten; (d) molybdenum. The middle inset (e) is a sideview image showing the growth of very thin tubes (<10 nm) among thicktubes. All scale bars are 100 nm.

FIG. 33. Size distribution of CNTs: (a) metal underlayer with amorphousSi layer; square—platinum—390 counts μm⁻²; circle—palladium—226 countsμm⁻²; up-triangle—tungsten—212 counts μm⁻²; downtriangle—molybdenum—89counts μm⁻² and (b) metal underlayer without amorphous Si layer;square-molybdenum-5 counts μm⁻²; circle—tungsten—73 counts μm⁻².

FIG. 34: Equivalent circuit diagram of the electrical measurements: (a)metal-metal configuration; (b) metal-CNT configuration; (c) CNT-CNTconfiguration.

FIG. 35: (a) I-V characteristics of metal underlayers for CNT-metalconfiguration on samples with an amorphous Si layer; inset: the samemeasurements for samples without the Si layer. (b) Conductancedeviations for samples with the amorphous Si layer, plotted in log-logscale. The straight dotted line represents the metal-metal conductancefor different metal underlayers. Current is dominated by surface leakageif the conductance value is above the dotted line and poor contacts areconsidered if it is below the dotted line. Circle-metal-metalconfiguration; square-CNT-CNT configuration; triangle-CNT-metalconfiguration.

FIG. 36: SEM micrograph of grown fibers on a W metal underlayer. (a)Represents the fibers grown from 100 nm dots with 500 nm pitch. Allcatalyst dots nucleated for growth of more than one fiber. Inset showsno break up of the catalyst after heating. (b) After growth when Nicatalyst was deposited on W directly. No growth is observed. (c) Fibersgrown from prefabricated 50 nm dots with 1 μm pitch. Most of the dotsnucleated to grow individual fibers. (d) Individual fibers grown from 50nm prefabricated catalyst dots with 500 nm pitch.

FIG. 37: SEM micrograph of grown fibers on Mo metal underlayer. (a)Represents the fibers grown from a film of Ni/a-Si catalyst layer. (b)Grown fibers from a 2 μm catalyst stripe. Inset picture is taken fromthe middle of the stripe. (c) Fibers grown from prefabricated 100 nmdots. Most of the dots nucleated to grow more than one fiber. (d)Individual fibers were grown from 50 nm prefabricated catalyst dots.

FIG. 38: Sequential presentation of the results at different stages ofthe fabrication procedures: (a) after lithography and metal depositionwhere 1200 μC cm−2 dose was applied, (b) after an annealing step beforegrowth of CNF. A high resolution image of a dot is shown in the inset(c) after growth of CNFs at 700° C. for 20 min (from 60° tiltedsubstrates) and (d) after a growth step of CNFs where no intermediateamorphous Si layer was applied, resulting in no growth of CNFs.

FIG. 39: Diameter as a function of dose for dots after the lithographystep. A linear fit of the measured values is indicated by a straightline.

FIG. 40: SEM micrograph of the grown CNFs for dose scale 800 μC cm⁻² forthree different metal underlayers. The column corresponds to 1 μm and500 nm pitch respectively. Micrographs are taken from 60° tiltedsubstrates. All scale bars are 1 μM.

FIG. 41: SEM micrograph of the grown CNFs for dose scale 1200 μC cm⁻²for three different metal underlayers. The column corresponds to 1 μmand 500 nm pitch respectively. Micrographs are taken from 60° tiltedsubstrates. All scale bars are 1 μm.

FIG. 42: Tip diameter of grown CNFs as a function of the catalystdiameter. Error bars indicate the standard deviation from the averagevalue. The trend of the average value is indicated by a dashed-dottedline for the W substrate.

FIG. 43: Average length distribution is plotted as a function of thecatalyst diameter for different metal underlayers. Error bars representthe corresponding standard deviation.

FIG. 44: Pitch induced limitations on high density growth of CNFs: (a)no predominant catalyst cluster conglomeration is present after theannealing step (top view), and (b) forest-like growth of CNFs aftergrowth resembles the growth from the film of the catalyst (from 60°tilted substrate).

DETAILED DESCRIPTION Overview

The present invention is directed to processes for makingnanostructures, singly, or in arrays, on a conducting substrate. Inparticular, the processes of the present invention permit choices ofmaterial, and sequences of materials, lying between the substrate andthe base of the nanostructure, to control various properties of theinterface between the nanostructure and the substrate, properties of thebody of the nanostructure, and the composition of the tip of thenanostructure. It is preferable that the nanostructures form columnsthat grow perpendicularly, or almost perpendicularly up from thesubstrate. However, this does not exclude the possibility to grow thenanostructures at other angles from the substrate such as on thesubstrate, (i.e., parallel to the substrate), or at an inclined angleother than 90°.

Accordingly, the present invention relates to: a method ofgrowing/depositing nanostructures utilizing existing CMOS technology; amethod of growing nanostructures on CMOS compatible conductingsubstrates and glass substrate and flexible polymer substrates used inareas that utilize thin film technology; a method to control thechemical interactions and hence controlling the end chemical compoundsin the nanostructures; and a method to control the chemical reactions byhaving multilayer material stacks consisting of at least oneintermediate layer between the substrate and a catalyst layer, whereinthe intermediate layer is not of the same material as either thecatalyst layer or the conducting substrate.

The present invention therefore provides a method for integratingnanostructures into CMOS technology and to achieve downscaling, highercomponent density and new functionality in, e.g., integrated circuits.

The ability to grow nanostructures on different metal underlayers (metalsubstrates) is important for several other reasons, including the factthat the identity of the metal is an additional parameter that can betuned to control the parameters of grown nanostructures such as height,diameter, density, etc., and because different metal work functions canbe exploited to control the height of the Schottky barrier between themetal underlayers and the nanostructures, thus permitting control overdevice functionality.

By controlling the choice of material stacks and the sequence ofdifferent materials, the layers in a stack can be used to controlproperties of the grown/deposited nanostructures.

In particular, by varying the materials and sequence of the materialsthe properties of the following can be controlled: the interface betweenthe nanostructure and the substrate can be controlled to have propertiesthat include, but are not limited to, Ohmic barriers, Schottky contacts,or controllable tunneling barrier(s); the body of the nanostructures;and the chemical composition of the tip of the nanostructures.

By controlling the properties of these three parts (the interface, thebody, and the tip), different structures, components and devices can befabricated which can be used in different applications. By controllingthe properties of these three parts in combination with differentstructures, components and devices, different functionality can beachieved. For example, the tip of the nanostructure can be tailored tohave a particular chemical property, or composition. Such tailoringpermits the tip of the nanostructure to be functionalized in differentways.

Nanostructures

The nanostructures formed by the methods of the present invention arepreferably made predominantly from carbon. However, other chemicalcompositions are consistent with the methods of the present inventionand are further described herein.

Nanostructures as referred to herein, encompass, carbon nanotubes,nanotubes generally, carbon nanostructures, other related structuressuch as nanofibers, nanoropes, and nanowires, as those terms areunderstood in the art.

By carbon nanotube (CNT), is meant a hollow cylindrical molecularstructure, composed principally of covalently bonded sp²-hybridizedcarbon atoms in a continuous network of edge-fused 6-membered rings, andhaving a diameter of from about 0.5 to about 50 nm. Typically a nanotubeis capped at one or both ends by a hemispherical carbon cap having fused5- and 6-membered rings of carbon atoms, though the nanotubes of thepresent invention are not necessarily capped. Carbon nanotubes may be,in length, from a few nanometers, to tens or hundreds of microns, toseveral centimeters.

The typical make-up of a CNT is analogous to a sheet of graphitic carbonwrapped on itself to form a closed surface, without any dangling bonds.Thus, CNT's typically consist of a closed network of 6-membered carbonrings, fused together at their edges. Most CNT's have a chirality thatcan be envisaged as arising if a sheet of graphitic carbon is shearedslightly before it is bended back on itself to form a tube. CNT's of anychirality may be formed by the present invention. It is also consistentwith the present invention, however, that the carbon nanotubes also mayhave a number of 5-membered rings, fused amongst the 6-membered rings,as is found in, for example, the related “fullerene” molecules, andwhere necessary to, for example, relieve strain or introduce a kink.Carbon nanotubes have electrical properties that range from metallic tosemiconductors, depending at least in part on their chirality.

By suitable choice of materials lying in between the substrate and thebase of the nanostructure, and their sequence, the morphology of thenanostructure that is formed can be tailored. Such nanostructuresinclude, but are not limited to, nanotubes, both single-walled andmulti-walled, nanofibers, or a nanowire. Such tailoring can arise from,e.g., the choice of texture of the catalyst layer that is positionedbetween the substrate and the nanostructure.

Carbon nanotubes made by the methods of the present invention may be ofthe single-walled variety (SWCNT's), having a cylinder formed from asingle layer of carbon atoms such as a single layer of graphitic carbon,or of the multi-walled variety (MWCNT's), having two or moreconcentrically arranged sheaths of single layers. MWCNT's may consist ofeither concentric cylinders of SWCNT's or stacks of frusto-conicalshaped single-walled structures.

A carbon nanofiber (CNF) is typically not hollow, but has a“herring-bone” or “bamboo”-like structure in which discrete segments ofcarbon fuse together one after another. The typical diameters rangesfrom 5 nm to 100 nm. A conical segment of catalyst containing materialis typically found at the tip of such a nanofiber. Carbon nanofibers arethus not crystalline and have different electrical conductivity fromcarbon nanotubes. Carbon nanofibers are effective interconnects inelectronic circuits because they support electric current densities ofaround 10¹⁰ A/cm². Carbon nanofibers thus have a higher atomic density,given by numbers of carbon atoms per unit volume of fiber, than the,hollow, nanotubes.

Carbon nanofibers made according to the present invention also can begenerally straight, and have a conical angle <2°, see FIG. 1, where theconical angle definition assumes that the base of the nanostructure isbroader than its tip. Since an angle θ≈ tan θ when θ is small, theconical angle≈(w_(b)-w_(t))/2 L, where w_(b) and w_(t) are the width ofrespectively the base and the tip of the nanostructure, and L is itslength, measured along its axis.

A carbon nanorope has a diameter in the range 20-200 nm, and thus istypically larger in diameter than a carbon nanotube. A carbon nanoropeis typically constructed by intertwining several nanotubes in a mannerakin to the way in which a macroscopic rope consists of several strandsof fiber bundled together. The various nanotubes in a nanorope may betwisted around one another or may line up substantially parallel to oneanother; the individual nanotubes are held together principally by vander Waals forces. Such forces, although individually weaker than acovalent bond between a pair of atoms, are in the aggregate very strongwhen summed over all of the pairs of atoms in adjacent tubes

The Interface

According to the present invention, by suitable choice of materials andtheir sequence, the interface between the base of the nanostructure andthe substrate can be chosen to have various electrical properties. Forexample, it can be chosen to be an ohmic contact, a Schottky barrier, ora controllable tunnel barrier.

An Ohmic contact is a metal-semiconductor contact with very lowresistance, independent of applied voltage (and which may therefore berepresented by a constant resistance). The current flowing through anohmic contact is in direct proportion to an applied voltage across thecontact as would be the case for an ohmic conductor such as a metal. Toform an ohmic contact, the metal and semiconductor must be selected suchthat there is no potential barrier formed at the interface (or so thatthe potential barrier is so thin that charge carriers can readily tunnelthrough it).

A Schottky barrier is a semiconductor-metal interface in which themetal-semiconductor contact is used to form a potential barrier.

A tunnel barrier is a barrier through which a charge carrier. such as anelectron or a hole, can tunnel.

FIG. 2 is a flow-chart that describes in overview a process of makingnanostructures on a substrate according to the present invention. First,one chooses a stack material, step 10. Then, a stack is created from thechosen materials, step 20, for example by deposition, sputtering orevaporation on to a substrate. Then, nanostructures are grown on thestack, step 30, for example in a growth/deposition chamber. Finally, thestructure is incorporated into a device, by one or more additionalfabrication techniques, step 40.

Chemical Vapor Deposition (CVD) is the preferred method for growth ofnanostructures according to the present invention. However, there aredifferent kinds of CVD methods that can be used, e.g., thermal CVD,PECVD, RPECVD, MOCVD (metallo-organic CVD), etc. It would be understoodby one of ordinary skill in the art, that other variants of CVD arecompatible with the present invention and that the practice of thepresent invention is not limited to those methods previously referenced.

It is preferable that the substrate for use with the present inventionis a conducting substrate. Accordingly, it is preferably a metal, or ametal alloy substrate.

By the methods of the present invention, step 10 can influence theproperties of the nanostructures that are grown. In particular, thenature and properties of the nanostructure are governed by the natureand extent of interdiffusion of the layers between the substrate and thenanostructure. Permitting interdiffusion can control the diameter andmorphology of the nanostructure, the number of nanotubes that grow perunit area of substrate, as well as the density of a nanostructure, andthe electrical properties of the interface. On the other hand, usingmaterials that impede diffusion between the substrate and the carbonnanostructure can control chemical interactions with the interfacematerials on both sides of the material, as well as the electricalproperties of the interface.

The layers of materials in the stack can be deposited as a continuousfilm in the case where it is desired to grow many, e.g., an array ofseveral hundreds or many thousands of, nanostructures on a singlesubstrate. A patterned film can also be used to control the propertiesbut in specific localized areas, leading to fabrication of individualdevices. The deposited film thickness may vary from 0.5 nm to more than100 nm, e.g., as much as 150 nm, 200 nm, or even 500 nm, depending onthe substrate underneath. Preferably, however, the thickness is from 1to 10 nm, and even more preferably, from 5 to 50 nm.

The nanostructures of the present invention can also be grownindividually rather than as a dense “forest”. For example, suchnanostructures may be discrete carbon fibers. This is the case wherecatalyst layer and sizes are defined by lithography for example. For thecase where a continuous film (in the form of stripes and squares largerthan 100 nm×100 nm) is used, more densely packed structures are possible(approximately 15 nm spacing between two adjacent nanostructures ispreferred). In such continuous film configurations, the packing densityand resulting diameter of the nanostructures can however be controlledby the choice of support layer.

In particular, the body of the nanostructures can be designed to bestructures that include: hollow with electrical properties such assemiconducting or metallic; not hollow with different electricalproperties (mainly metallic); hollow with different mechanicalproperties; and not hollow with different mechanical properties.

Controlling Nanostructure Properties

The present invention encompasses nanostructures grown from substrates,and interface layers situated therebetween, having the followingcharacteristics. The substrate is preferably a metal layer, which may bedisposed on a support. The support is typically a wafer of silicon orother semiconducting material, glass, or suitable flexible polymer usedin thin film technology. The metal is preferably selected from the groupconsisting of molybdenum, tungsten, platinum, palladium, and tantalum.The thickness of the metal layer is preferably in the range 1 nm to 1 μmand even more preferably in the range 1 nm to 50 nm. The metal layer ispreferably deposited by any one of several methods known in the art,including but not limited to: evaporative methods such as thermal orvacuum evaporation, molecular beam epitaxy, and electron-beamevaporation; glow-discharge methods such as any of the several forms ofsputtering known in the art, and plasma processes such asplasma-enhanced CVD; and chemical processes including gas-phaseprocesses such as chemical vapor deposition, and ion implantation, andliquid-phase processes such as electroplating, and liquid phase epitaxy.Examples of deposition technologies are found in Handbook of Thin FilmDeposition, K. Seshan, Ed., Second Edition, (William Andrew, In., 2002).

The interface layers, also called intermediate layers or an intermediatelayer, comprise one or more layers, in sequence, disposed upon thesubstrate. On top of the interface layers is a layer of catalyst. Thenanostructure is grown from on top of the catalyst layer.

The interface layers may consist simply of a single layer of material.In this circumstance, the single layer is preferably silicon orgermanium. The layers can be deposited in the form of amorphous orcrystalline by techniques such as evaporation, or sputtering. Thepreferable thickness ranges from 1 nm to 1 μm and even more preferablyin the range 1 nm to 50 nm.

The interface layers may comprise several layers of different materialsand may be, arbitrarily, classified according to function. For example,the layers in the vicinity of the substrate are characterized as layersthat influence the electrical properties of the interface. The layers inthe vicinity of the catalyst are characterized as layers that influencethe composition and properties such as electrical/mechanical propertiesof the nanostructure.

Various configurations of interface layers are compatible with thepresent invention. For example, a sequence of up to 3 layers may bedeposited on the substrate, for the purpose of controlling theelectrical properties of the interface. Such configurations include, butare not limited to: a sequence of insulator, conductor or semiconductor,and insulator; a sequence of insulator adjacent to the substrate, and asemiconducting layer; a sequence of semiconductor, insulator,semiconductor; a sequence of two insulating barrier layers adjacent tothe substrate, and a semiconductor; a single layer of a metal that isdifferent from the metal of the substrate; and a sequence of a metalthat is different from the metal of the substrate, and a semiconductinglayer. In such configurations, the insulator may be selected from thegroup consisting of: SiO_(x), Al₂O₃, ZrO_(x), HfO_(x), SiN_(x), Al₂O₃,Ta₂O₅, TiO₂, and ITO. The semiconductor may be silicon or germanium. Themetal, where present, may be palladium, platinum molybdenum or tungsten.Where two layers of the same character are present, e.g., twosemiconducting layers, it is not necessary that the layers have the samecomposition as one another.

The uppermost layer of the foregoing interface layers may itself abutagainst the catalyst layer. This is particularly the case where theuppermost layer is a semiconductor such as silicon or germanium.However, it is additionally possible for the foregoing interface layersto have disposed upon them a further layer or sequence of layers thatlies between them and the catalyst layer. Such additional, or second,interface layers are thought of as controlling the properties andcomposition of the nanostructure. The second interface layers may be apair of layers, such as a metal layer and on top thereof a semiconductorlayer adjacent to the catalyst layer. Alternatively, the secondinterface layers may simply consist of a single layer of semiconductor.The metal layer, where present in the second interface layers, ispreferably selected from the group consisting of tungsten, molybdenum,palladium, and platinum. The semiconducting layer in the secondinterface layers is preferably silicon or germanium.

The catalyst layer is typically a layer of metal or metal alloy, and maycontain very fine particles of metal or metal alloy instead of being acontinuous film. The catalyst layer preferably comprises a metalselected from the group consisting of nickel, palladium, iron,nickel-chromium alloy containing nickel and chromium in any proportions,and molybdenum.

The invention is primarily focused on a multi-stack configuration of atleast one material layer between the catalyst layer and the conductingsubstrate, wherein the material is not of the same kind as the catalystand conducting substrate, and wherein the material controls the chemicalreactions between the various layers. Thus, the growth of thenanostructures on different conducting substrates can be controlled.Thereby the morphology and properties of the grown structures as well asthe tip materials of the grown structures can be controlled. The currentinvention can be extended to having several stacks of materials ofdifferent kinds (semiconducting, ferroelectric, magnetic, etc.) whichcan be used to control the properties at base/interface, body and thetip of the nanostructure. It is also possible that the nanostructure isgrown upon a conducting layer which is itself deposited on a substratethat itself can be of any kind, such as conducting, insulating orsemiconducting.

High-k dielectric materials are mainly used as gate materials for CMOSdevices. In the present invention such materials are utilized in part inmulti-layer stacks to define the properties of the grown nanostructureas well as to control the interface properties between the nanostructureand the conducting layer.

According to the methods of the present invention, the presence of twoor more intermediate layers will influence the texture/crystallographicstructures of each other and the final catalyst particles.

Accordingly, the present invention preferably includes a conductinglayer, at least one intermediate layer directly on the conducting layer,at least one catalyst layer directly on the intermediate layer, and ananostructure on the catalyst layer.

The substrate may be a disposed on a support commonly used insemiconductor processing, such as a silicon wafer, or oxidized siliconwafer. The support may alternatively be a glass or metal or thinflexible polymer film used in the thin film technology as substrate.

It is to be understood that the at least one intermediate layer ischosen to control various electrical properties of the interface betweenthe substrate and the carbon nanostructure.

It is further to be understood that the choice of at least one catalystlayer controls various properties of the carbon nanostructure.

The grown nanostructures are preferably carbon-based materials such ascarbon nanotubes (CNT), and carbon nanofibers (CNF). Carbonnanostructures form when the entire structure is placed in a mixture ofcarbon-containing gases. Preferred gases are hydrocarbons such as CH₄,C₂H₂ and C₂H₄, and generally aliphatic hydrocarbons having 5 or fewercarbon atoms, of any level of saturation.

The nanostructures can also be of different semiconducting materialsreferred to as III-V, or II-VI materials, such as InP, GaAs, AlGaAs,depending on the choice of catalyst and subsequent chemical chamberconditions used. Keeping all the other materials stack same as for acarbon nanostructure described herein, simply changing the catalyst typeand or composition of gases can facilitate growth of these non-carbonnanostructures. Therefore without deviating from the other aspects ofthe invention described herein, a person of ordinary skill in the artcan grow different kinds of solid state nanostructures. Examples ofconditions for forming such nanostructures are as follows.

SiC nanostructures: chambers—MOCVD (metallo organic CVD); gascomposition—dichloromethylvinylsilane [CH₂CHSi(CH₃)Cl₂]; catalyst—Ni;and temperature: 800-1200° C.

Si nanostructures: chamber type—vapor-liquid-solid (VLS)/CVD; gascomposition—SiH₄, Si₂H₆; catalyst—Ni; and temperature 500-1000° C.

InP/GaP nanostructures: chambers—MOCVD/CVD; gas composition—elementalindium and gallium with triphenyl phosphine, trimethyl-gallium and N₂;catalyst; and temperature: 350-800° C.

GaN nanostructures: chambers—MOCVD (metallo organic CVD);gas/composition—elemental gallium and ammonia gas; catalyst—Ni; andtemperature: 800-900° C.

ZnO nanostructures: chambers—MOCVD/CVD; gas composition—oxidation ofZinc carrying elements; catalyst—Ni; temperature 30-700° C.

The grown nanostructures for materials other than carbon can be of theform of forests consisting of uniform structures covering the substratearea and/or arrays, or individual structures.

The choice of catalyst plays an important role because the growth ofcarbon nanostructures is ordinarily catalytically controlled. Since thecrystallographic orientation of the catalysts partakes in defining themorphology of the nanostructure, it is expected to obtain differentgrowth mechanisms from different types of catalysts. Besides catalystcrystallographic orientation, there are many other growth conditionsthat influence the structure formation, such as the mixture of gases,current density for the case when plasma density is controlled, voltagebetween the cathode and anode, temperature of the substrate, chamberpressure, etc. (see, e.g., Kabir, M. S.; Morjan, R. E.; Nerushev, O. A.;Lundgren, P.; Bengtsson, S.; Enokson, P.; and Campbell, E. E. B.,Nanotechnology 2005, (4), 458) incorporated herein by reference).

FIGS. 3A and 3B show an overview of various structures according to theinvention. FIG. 3A shows how a carbon nanostructure having a tip 110,body 120 and a base 130, and made by processes described herein, can bepositioned vertically on a metal substrate as in the left-hand side ofFIG. 3A, or horizontally on an insulating substrate as in the right-handside of FIG. 3A. Positioning on an insulating substrate will allow forfurther processing for making functional devices. The bottom substrate(not shown) underneath the insulating layer can be used as a bottom gatedielectric and the substrate underneath the oxide as bottom gateelectrode to e.g., modulate the resistance of a semiconductingnanostructure. See FIG. 3B.

FIG. 3B shows various configurations of one or more intermediate layers210 between a conducting substrate 200 and a catalyst layer 220. Theinvention proposes a platform comprising at least one material stack(denoted, e.g., layer 1) between the catalyst layer and the conductingsubstrate. The purpose of the multiple materials stacks (denoted, e.g.,layer 1, layer 2, . . . layer n) is to control the interface propertiesbetween the conducting substrate and the grown nanostructures (forexample, ranging from Ohmic contact to Schottky barrier), the propertiesof the grown nanostructures (morphology, mechanical, and electricalproperties), and the properties of the tip 110 of the grownnanostructures.

FIGS. 5 and 6 show embodiments of a device having a single intermediatelayer. In FIG. 5, in another embodiment, a metal layer 510 is on a wafer520; an intermediate layer of silicon 530 is on the metal layer; and acatalyst layer 540, typically Ni, or Fe, or others such as NiCr or, Pd,is on the intermediate layer. Together, layers 530 and 540 are referredto as the interface.

In FIG. 6, another typical individual nanostructure is shown. In thisstructure, a metal layer 610 is on a wafer 620; an interface 630 betweenthe metal layer and a body of a nanostructure 640 is formed from anintermediate layer of semi-conducting material 645 such as silicon. Thetip 650 of the nanostructure contains a mixture of materials, includingprincipally catalyst that has diffused up the body of the nanostructureas the nanostructure has grown, and also some metal.

FIG. 4 shows a representative embodiment having a multilayer stacksupporting a partially formed nanostructure 499. A metal layer 410 actsas a substrate, and is disposed on a support 420, e.g., a wafer ofsilicon. A 3-layer stack acts as an intermediate layer between the metalsubstrate and a second stack of catalytic layers and controls theelectrical properties of the interface. The intermediate layer has, inorder, starting with a layer in contact with the metal: a first controllayer 430, of e.g., SiO_(x), or Al₂O₃; on top of the first control layeris a metal/semi-metal layer 440, e.g., Ge; on top of themetal/semi-metal layer is a second control layer 450 of, e.g., ZrO_(x)or HfO_(x) or any other material with high k dielectric value such asSiN_(x), Ta₂O₅, Al₂O₃, and TiO₂. The subscript ‘x’ in a chemical formuladenotes a variable stoichiometry, usually controllably variable. The twocontrol layers control diffusion from respectively the metal/semi-metallayer into the substrate and into the catalyst stack. The thickness andcomposition of the two control layers provide two variables with whichsuch control may be achieved. The thickness for a single layer rangesfrom less than 10 nm to several hundreds of nanometer and the thicknessof the total material stack ranges from less than 10 nm up to micronsand above. Together, the first control, metal/semi-metal, and secondcontrol layers permit control of electrical properties of the interfacebetween the metal and the carbon nanostructure. To obtain differentelectron/hole tunneling properties, it is a matter of choosing differentoxides to give a variation of electrical tunneling properties and hencevarying electrical properties of the interface between the nanostructureand the base substrate 410. Principally, such choices are determined bythe dielectric constant of the control layer materials such as oxides.

Also referring to FIG. 4, a multilayer stack disposed on the secondcontrol layer controls properties of the carbon nanostructure that growsabove it. In the example shown, adjacent to the second control layer isa first metal layer 460, e.g., tungsten, molybdenum, palladium,platinum; adjacent to the first metal layer is a silicon layer 470; andon top of the silicon layer is a second metal layer 480 composed of,e.g., nickel or palladium.

FIG. 7 shows another embodiment of a nanostructure having a tip 610, abody 620, and an interface 630. A metal layer 640 is disposed on a wafer650 and consists of a metal selected from the group consisting ofmolybdenum, tungsten, platinum, tantalum, and palladium. A two-layerinterface 630 is on the metal layer 640 and has a first intermediatelayer 660 of oxide, such as SiO_(x), ZrO_(x), HfO_(x), or TiO_(x); asecond intermediate layer 670, composed of silicon, is disposed on thefirst intermediate layer and is in contact with the body of thenanostructure. The tip 610 of the nanostructure contains Ni, Fe, Mo, orPd, or an alloy such as NiCr or a mixture of the materials found in thematerial stack. The metal content of the tip originates with a layer ofcatalyst (not shown in FIG. 7) that was situated between the uppermostintermediate layer and the bottom of the nanostructure.

FIG. 8 shows another nanostructure having a tip 710, a body 720, and aninterface 730 which comprises a multi-layer stack. A metal layer 740 isdisposed on a wafer 750. A three-layer interface 730 is on the metallayer 740 and has a first intermediate layer 760 of semi-metal such asgermanium; a second intermediate layer 770 of oxide, such as SiO_(x),ZrO_(x), HfO_(x), or TiO_(x); and a third intermediate layer 780,composed of silicon, which is in contact with the body of thenanostructure. The tip of the nanostructure contains Ni, Fe, Mo, or Pd,or an alloy such as NiCr or a mixture of the materials found in theinterface.

FIG. 9 shows another embodiment of a nanostructure: a metal layer 910 isdisposed on a wafer 920; an interface 930 having three intermediatelayers is disposed on the metal layer 910. The three intermediatelayers, in sequence moving away from the metal, are: a second barrierlayer 940, a first barrier layer 950 and a semiconducting layer 960, incontact with the body of the nanostructure 970. The first barrier layercan be used as a material diffusion barrier upwards/downwards, and thesecond barrier layer can be used as defining the electrical tunnelbarrier. The body of the nanostructure can have electrical propertieseither as a semiconductor or a conductor. The tip 980 of thenanostructure contains catalyst.

As is seen from FIGS. 6-9, catalyst diffuses into the body of thenanostructure during growth initiation. This process is described infurther detail in FIG. 10. In FIG. 10, a metal underlayer 1010 of ametal such as W, Mo, Pt, Pd, is on a wafer 1020. An intermediate layerof a semiconducting material 1030 such as silicon or germanium, or acompound of III-V elements from the periodic table, is on the metalunderlayer. A catalyst layer 1040 having a metal such as Ni, Fe, Co, oran alloy such as NiCr is on the intermediate layer.

A stage during growth of the nanostructure is shown in the right-handpanel of FIG. 10. An expanded view of the metal underlayer is shown. Aninterface 1060 between the metal underlayer and the body 1050 of thegrowing nanostructure contains an alloy of catalyst with metalunderlayer, metal silicides, and the metal underlayer itself.

The intermediate layer 1030 is used to start the growth process. Howeverit diffuses into the metal underlayers creating metal compounds such asmetal-silicides if the intermediate layer is silicon, which function asOhmic contacts with the metal underlayer. Accordingly the nanostructureis grown by direct contact with metal underlayer where no intermediatelayer is present in between the initial catalyst and metal underlayer. Asmall portion of catalyst is present at the bottom. The tip consists ofcatalyst rich metal underlayer: a large portion of catalyst is presentat the tip of the nanostructure together with a small portion of metalunderlayer.

In FIG. 11, an embodiment of nanostructure growth uses a tungsten (W)metal underlayer 1110 on a wafer 1120. A stack having a layer of silicon1130 on top of the metal underlayer, and a layer of nickel 1140 on topof the silicon is in contact with a growing nanostructure 1180. Thematerial stack conditions before growth (FIG. 11, left hand panel) showdiscrete layers. The material stack conditions after growth (FIG. 11,right hand panel) show that interdiffusion amongst the layers hasoccurred: there are now distinct regions of nickel-tungsten alloy 1150,tungsten-silicon alloy 1160, and undiffused tungsten 1170. It is alsoconsistent with the conditions that the regions of, e.g., nickel andtungsten have a gradation of properties without a discontinuity in theconcentrations of the respective metals or a sharp concentrationgradient.

FIG. 12 shows a multilayer stack between a metal underlayer 1210 and ananostructure body 1230. The multilayer stack comprises two interfaces,a first interface 1240 to control electrical properties of theinterface, and a second interface 1250 to control physical properties ofthe nanostructure body. Metal underlayer 1210 is on a wafer 1220. Firstinterface 1240 comprises two layers disposed on the metal control theelectrical properties of the interface. A layer of germanium 1260 isdirectly on the metal 1210, and a layer 1270 of an oxide such asSiO_(x), ZrO_(x), HfO_(x), or TiO_(x) is directly on the germanium. Theoxide layer acts as a buffer. Two further layers, disposed on the oxidelayer, serve to control physical properties of the body of thenanostructure. A first layer 1280 of silicon is directly on the oxidelayer, and a layer 1290 of metal catalyst such as nickel, iron, orpalladium is in between the silicon layer and the body of thenanostructure.

Process for Forming Nanostructures

The present invention further comprises a process for formingnanostructures. The process comprises first depositing an electrode on asubstrate. The substrate, as further described herein, may be a wafer ofsilicon, and preferably has an insulating coating, such as an oxide, forexample SiO₂. The electrode functions as an underlayer for thenanostructure, and is made of a conducting material, preferably a metalsuch as molybdenum, niobium, or tungsten. The method of depositing theelectrode can be any one familiar to one of ordinary skill in the art,but is preferably a method such as electron beam evaporation. Theelectrode layer is between 10 and 100 nm thick, and is preferably 50 nmthick.

Optionally, a resist is then deposited on the electrode layer. Such aresist is usually used for technologies that utilize lift-off processesfor metal depositions. An exemplary resist is a double-layer resistconsisting of 10% co-polymer and 2% PMMA resist, that is applied byconsecutive spin coating and baking. The resist is thenpatterned/exposed by a radiation source, such as UV light or an electronbeam, to transfer the design into the resist layer.

A catalyst layer, either as a sheet or as dots, is fabricated on themetal substrate or on the resist, where present. Dots of catalystfacilitate controlled growth of individual nanostructures in preciselocations. Catalyst dots may be constructed by electron beamlithography. Their dimensions can be controlled using the shotmodulation technique. With this technique, catalyst dot sizes can bedetermined with nanometer precision, and dots as small as 5-10 nm indimension can be formed. The catalyst layer is not heated during thisstage.

On the catalyst layer, layers of other materials are deposited. Suchlayers include at least one layer of semiconducting material and mayinclude at least one layer of a metal different from the metal of theunderlying electrode. The semiconducting material is preferablydeposited using an electron beam evaporator. The semiconducting materialis preferably amorphous silicon, and the layer has a thickness of 5-100nm, preferably 10 nm.

After the various layers, including one layer of semiconductingmaterial, are deposited a layer of catalyst material is deposited,thereby forming an uppermost layer upon which nanostructures areultimately fabricated. The catalyst layer is deposited by standardtechniques known in the art such as electron beam evaporation orsputtering.

Optionally, if a resist has been applied, it can now be removed by alift-off process, for example by washing the structures in acetone at60° C., followed by washing with iso-propyl alcohol. After thesewashings, the structures are rinsed in deionized water and blow-driedwith nitrogen gas.

Nanostructures can now be grown upon the remaining areas where catalystlayers are exposed. The preferred technique for effecting such growth isplasma-enhanced chemical vapor deposition. As previously describedherein, the composition of the vapor will determine the types ofnanostructures that are grown. For example, carbon nanotubes can begrown at 5 mbar pressure in a (1:5) mixture of C₂H₂:NH₃ gas. Growth ofnanostructures typically occurs at high temperatures, in the range600-1,000° C., such as 700° C. The substrate (with electrode,semiconducting material, and catalyst layers thereon) are brought up tosuch high temperatures by ramping the temperature up relatively rapidly.Exemplary rates are from 1-10° C./s, preferred rates being in the range3-6° C./s. Such conditions have been referred to in the art as‘annealing’, and preferably occur in a vacuum. A low vacuum (e.g.,0.05-0.5 mbar pressure) suffices. The source gases for thenanostructures are introduced into the chamber when the maximumtemperature is reached.

The nanostructures are typically cooled to room temperature before theyare permitted to be exposed to air.

Control over individual nanostructure formation is thus achieved becausespecifically tailored catalyst dots are created, rather than relying onnon-uniform break up of a layer of catalyst by prolonged heating priorto nanostructure formation.

Applications

Applications of carbon nanostructures made by the methods describedherein include: construction of composites for use in structuralengineering, as well as structures having high strength but light mass,as might be used for objects sent into outer space; electrochemicaldevices and sensors for diagnostics, as are used in the life sciences;research tools such as electron emitters, small size X-ray generators,and atomic force microscopy probes; and applications to circuitrycomponents used in electronics such as interconnects, diodes, heatdissipative media, high frequency filters, optical devices such as lightemitting diodes, wave guides, opto-electronic circuits, hydrogen storagedevices, qubits for quantum computing, and super capacitors.

For example, FIG. 12 shows how an individual nanostructure can becomepart of an electric circuit. A 3-layer stack 1310 controls properties ofthe interface and consists of a first diffusion barrier 1330, adjacentthe metal 1320, and composed of SiO_(x) or Al₂O₃ or another dielectricmaterial. An island 1340, of metal or semi-metal is situated between thefirst diffusion barrier and a second diffusion barrier 1350 of ZrO_(x)or HfO_(x) or choice of other dielectric material. A further 3-layerstack 1360 controls properties of the interface and is on the seconddiffusion barrier layer 1350. A metal layer 1370 acts as a growthsubstrate to control the properties of the grown structures and is incontact with the second diffusion barrier 1350; a silicon layer 1380 ison the metal layer and a nickel or palladium catalyst layer 1390 is onthe silicon layer. The silicon layer allows interdiffusion to controlthe properties of the grown structures. The carbon nanostructure 1395 isconductive in this example and is a carbon nanofiber. Metal layer 1320is on a wafer 1305.

FIG. 14 shows how the nanostructure of FIG. 13 would function in anelectric circuit that contains a battery 1410 as a representativevoltage source. The items labeled 1330, 1340, 1350, and 1395 in FIG. 14correspond to similarly numbered items in FIG. 13.

FIG. 15 shows how an individual nanostructure can form part of anelectronic device. The metal underlying layer 1510, is disposed on wafer1520, and is for example tungsten and is a first instance of a firstmetal. A second metal, having a different work function from theunderlying metal, for example is platinum, forms a layer 1530 disposedon the underlying metal layer. This second metal layer controls theelectrical properties of the interface between metal layer 1510 and thecarbon nanostructure 1540. A second layer 1550 of the first metal isdisposed on the layer of second metal. This layer and the two above itcontrol the properties of the nanostructure. The two layers above layer1550 are, in sequence, silicon 1560, and iron 1570. The last of these isa catalytic layer. In this embodiment, the carbon nanostructure is asemiconductive carbon nanotube.

FIG. 16 shows how a nanostructure can form part of an electro-opticaldevice such as a light emitting diode, or as the variable conductingchannel like a transistor. A structure as in FIG. 15 is encapsulated inan insulator 1610 such as SiO_(x) on each side, and on top a third metal1620, such as tungsten or calcium, having a different work function thanthat of the bottom metal electrode 1630 in the figure. Metal layer 1510is disposed in a wafer 1630. Carbon nanostructure 1540 is asemiconducting carbon nanotube in this example.

FIG. 17 shows how an individual nanostructure can form a Schottkybarrier as part of an electrical device. A metal underlayer 1710 isdisposed on a wafer 1720. On top of the metal underlayer 1710, which iscomposed of, e.g., tungsten, is a pair of layers that controls theelectrical properties of the interface. A layer 1720 of a second metal,such as platinum, having a different work function from the metal of themetal underlayer 1710 is on top of the metal underlayer. A layer 1730 ofa semiconductor such as germanium is on the layer of second metal. Thiscombination of two metal layers and a semiconducting layer creates aSchottky barrier due to the mismatch of work function of differentmaterials and therefore controls the electrical properties of theinterface. Three further layers for controlling the properties of thenanostructure above, are disposed upon the semiconductor layer 1730. Insequence, the three layers are: a layer 1750 of the first metal (in thisinstance tungsten), a silicon layer 1760, and lastly a nickel layer 1770that functions as a catalyst. Disposed on the catalyst layer is ananostructure such as a carbon nanotube or a carbon nanofiber.

FIG. 18 shows how an individual nanostructure can form a Schottkybarrier as part of an electronic device. In this embodiment the lowersegment, mentioned in the foregoing paragraph in connection with FIG.17, and consisting of a metal and semiconductor layer, is excluded. Theremaining segment consisting of material layers determining that thenanostructure has semiconducting properties is present: a metal layer1820 is disposed on a metal underlayer 1810. The work function of themetal of layer 1820 is different from that for the metal underlayer1810. On layer 1820 are, in sequence, a semiconductor layer 1830, ofe.g., silicon, and a catalyst layer 1840 of e.g., iron. This embodimentconsists of a semiconducting nanostructure 1850 grown on metalunderlayer will create a Schottky barrier due to mismatch of workfunction of the metal and the band gap of semiconducting nanostructure.FIG. 19 shows a schematic representation of Schottky Barrier formationbetween the contact metal and the nanotube for a device as shown in FIG.18, according to two types of metal electrodes: (a) for a large workfunction metal, and (b) for small work function metal. In the formercase, the Fermi level E_(F) of the electrode metal is close in energy tothe valence band E_(V) of the carbon nanostructure (denoted SWCNT), andholes pass easily across the interface from the metal to the carbonnanostructure. In the latter case, the Fermi level of the electrodemetal is close to the conduction band E_(C) of the carbon nanostructure,and electrons pass easily across the interface from the metal to thecarbon nanostructure.

Field Emission Device

In particular, the nanostructures of the present invention may form thebasis of a field emission device. FIGS. 20A-20C show successivelydetailed views of such a device 2000. FIG. 20A shows, schematically, afield emission device having several pixels mounted in a chamber 2010.Each pixel has a metal substrate 2020, one or more nanostructures 2080mounted thereon. The metal substrate is in electrical communication witha metal cathode 2060 through one or more interconnects 2070. Cathode2060 is in electrical communication with a device controller 2050 and ananode 2040. Device controller is typically an electrical componentcapable of providing a voltage across a specific pixel. Devicecontroller is preferably able to control multiplexing of the device sothat pixels can be individually addressed, for example by using anactive addressing scheme. In normal operation, upon application of avoltage between the cathode and anode, nanostructures 2080 emitelectrons towards anode 2040. The electrons impact a phosphor layer 2030that is in contact with the anode, and cause it to emit one or morephotons of visible light. It is preferable that anode 2040 istransparent so that the photons are emitted in a direction away from thepixels. Chamber 2010 is preferably sealed so that it contains either avacuum or an inert gas such as argon. This arrangement ensures that thenanostructures have a long lifetime and do not decompose or react withoxygen or water vapor normally found in air.

The system depicted in FIG. 20A is more practical than cathode ray tubesused in the art because it can be flatter. It also offers brighterdisplays than other comparable displays used in the art, such as LED's,OLED's, and LCD's. For example, the contrast ratios achievable withLCD's are around 1,000:1, whereas those obtained from electron emissiondevices are around 20,000:1. Such contrast ratios make electron emittingdevices such as shown in FIG. 20A suitable for handheld devices such ascell-phones, GPS receivers, and other devices that see a lot of use inoutdoor lighting conditions.

FIG. 20B shows a more detailed view of an individual pixel from FIG. 20Ahaving electrical communication with device controller 2050.Nanostructure 2080 are optionally separated from each other by a layerof insulating material 2082. The thickness of the insulator is such thatthe tips of the nanostructures protrude from the upper surface of theinsulator, as shown. It is preferable that the nanostructures shown areof approximately equal length, preferably within 10% of each other, sothat equal amounts protrude from the insulator. Nanostructures 2080 arepreferably carbon nanotubes, and still more preferably single-walledcarbon nanotubes. In still other embodiments, nanostructures 2080 arenanofibers. Exemplary lengths of the nanostructures are 500 nm-10 μm,and exemplary diameters are 10 nm-100 nm. The pixel typically hasdimensions 10 μm by 10 μm and the nanostructures are typically spacedapart by 200 nm-1 μm. Thus, the number of nanostructures supported in agiven pixel is 100-2,500.

In practice for color displays, an individual pixel of a displaycomprises three of the structures shown in FIG. 20B, overlain with asuitable mask. Each of the three structures is masked to give one of thethree primary colors, red, green, and blue, and is independentlyaddressable for the purpose of generating a color image.

FIG. 20B also shows, schematically, a photon 2084 emitted from phosphorlayer 2030. Carbon nanostructures are effective light emitters becausethey give unidirectional emission towards phosphor layer 2030.

FIG. 20C shows a more detailed view of the base and interface layers ofan individual nanostructure 2080, for example in a pixel of FIG. 20B.Only the lower part of the body of nanostructure 2080 is shown. Acatalyst layer 2092 is shown in contact with the base of nanostructure2080. The catalyst layer 2092 is on a layer 2090 that may be a singlelayer of a semiconductor such as silicon or germanium, and may be amulti-layer stack of metals and or semiconductors as further describedherein. Layer 2090 is disposed upon metal substrate 2020. In theconfiguration depicted in FIG. 20C, the interface between metal 2020 andnanostructure 2080 forms an Ohmic contact.

The field emission device shown in FIGS. 20A-20C operates at a muchlower voltage than other comparable devices that are not grown on ametal substrate such as 2020, but instead are grown on an insulatingsubstrate. Metal 2020 is preferably tungsten, molybdenum, platinum orpalladium.

Electron Beam Writer

The nanostructures of the present invention may also form the basis ofan electron beam writer, as depicted in FIGS. 21-23. Such a device canbe viewed as a single-nanostructure version of the field-emission devicedescribed hereinabove. A device according to FIGS. 21-23 can findnumerous applications where a very fine, focusable beam of electrons isrequired. For example, it can be used in electron beam lithography tocreate nanometer scale lines (so called nanolithography). It can also beused in forms of electron microscopy, such as scanning electronmicroscopy, and in transmission electron microscopy.

FIGS. 21A-21C show cross-sectional views, viewed sideways, of anelectron-beam writing device 2100. Layer 2110 is a wafer, typically ofhighly doped silicon, which acts as a bottom electrode. Layer 2120 isinsulator such as silicon dioxide. Layer 2130 is also an insulator suchas SiO₂ which acts as a sacrificial layer that can be etched away duringthe manufacturing process of the electron beam writer. Layer 2140 is atop electrode, typically formed from a metal, and often referred to asan actuator electrode. A vertical free-standing nanostructure 2150resides in a cavity 2135 formed in layers 2130 and 2140. In someembodiments, nanostructure 2150 is disposed on a layer of top electrodemetal 2142. In other embodiments, nanostructure 2150 is disposed on thewafer 2110.

The embodiments shown in FIGS. 21A and 21B are of a single electrodestack electron-beam writer. The embodiment in FIG. 21C is of amulti-electrode stack device. In FIG. 21C, layers 2140 and 2160 aremetal electrode layers, and layers 2120, 2130 and 2170 are insulatinglayers all together to form the embodiment for a electron-beam writer.In FIG. 21C, layer 2160 acts as a gate to control the movement of thenanostructure 2150 and layer 2140 acts as an actuator electrode.

The embodiments described above as shown in FIGS. 21A, 21B and 21C canalso be used for making a relay switch (often referred to as anano-relay) where layer 2160 acts as the gate electrode to control themovement of the nanostructure 2150, the layer 2142 acts as the source ofthe device, and the layer 2140 in this example acts as the drain of thedevice. In this way, an embodiment of a three terminal device is formedwhere the nanostructure 2150 can be moved towards the drain layer 2140by applying electric field at the layer 2160.

The base of nanostructure 2150 is shown in detail in connection withFIG. 21C, though similar principles apply to any of the foregoingembodiments, in FIGS. 21A and 21B. Nanostructure 2150 is separated frommetal electrode layer 2142 by a catalyst layer 2152 and an adjacentinterface layer 2154. Interface layer 2154 may be a single layer, forexample of silicon or germanium, or may comprise multiple adjacentlayers. Where layer 2154 comprises multiple adjacent layers, at leastone such layer is silicon or germanium; the other layers may be othersemiconductors, insulators, or other metals different from the metal oflayer 2142, so as to give control over the properties of nanostructure2150.

Nanostructure 2150 is typically 500 nm-10 μm long from base to tip, andis preferably around 1 μm long. The diameter of the nanostructure istypically between 5 nm and 50 nm. Preferably nanostructure 2150 is acarbon nanostructure such as a carbon nanotube or nanofiber.

FIGS. 22A-22C show plan views from the top of various configurations ofelectrodes situated around a central vertical free-standingnanostructure 2150. In FIGS. 22A and 22B, multiple separatelycontrollable electrodes are envisaged, numbered 2140, 2141, and2143-2148. Exemplary numbers 4 and 8 are shown, though other numbers arepossible, depending on the degree of control that is desired of themotion of the nanostructure. For example, other numbers of electrodesinclude, but are not limited to, 2, 3, 5, 6, 10, 12, and 20.

In FIG. 22C, a single continuous electrode encircles the cavity 2135 inwhich nanostructure 2150 resides. FIG. 22D shows a perspective view ofthe embodiment in FIG. 22C.

In operation, a voltage selectively applied to electrodes 2140, etc.,can cause the tip of the nanostructure to move in space towards, or awayfrom, a particular electrode, due to the electric field created by theelectrodes. According to the disposition of the various electrodes,then, the nanostructure tip can move and therefore point in variousdirections. The directionality of the tip can therefore be controlled sothat electrons, when emitted from the tip in response to a suitableapplied voltage, will be caused to move in a desired direction.

FIG. 23 shows a schematic of an electron-writing device based on avertically-aligned free-standing nanostructure. The arrow across thenanostructure indicates a degree of freedom of motion within the planeof the figure. A beam of electrons, e⁻, is shown emanating from the tipof the nanostructure 2150 in the direction of a writing target, orsubstrate 2310, which also serves as an anode. Also shown in FIG. 23 isa schematic electrical circuit between the anode and the top electrode.

In certain embodiments, it is possible to change the direction of thebeam direction after it has been emitted from the nanostructures,instead of or in addition to altering the beam direction by causing thenanostructure to tilt in the desired direction. In which case, thedirection of the beam after emission can be controlled by an electronoptical system (EOS), based on, for example, magnetic lances.

FIG. 24 shows an alternative embodiment in which a nanostructure 2410 issupported horizontally and has at least one degree of freedom, as shownby the arrow, to move in a vertical plane. An electrode 2420 in contactwith the nanotubes communicates electrically with anode 2430, which alsoacts as a writing target.

The electron beam writer described herein may be tailored to variousapplications by appropriate choices of the various materials. Forexample, the support wafer 2110, and the insulator disposed thereon 2120may be varied, as may the choice of metal for the electrodes. The mannerof growth of the nanostructure, as further described herein, may permitfunctionalization of the nanostructure tip, as well as its morphology.

EXAMPLES Example 1 Electron-Beam Writer

FIG. 25 shows a SEM image of an embodiment of an electron beam writerthat may be used as a nano writer, wherein: D_(CNT)=Diameter ofCNT/CNF/nano-structure; L_(SD)=thickness of insulator; L_(CNT)=Length ofCNT/CNF/nanostructure; L_(g)=Distance between CNT/CNF/nanostructure andelectrodes; F_(Elas)=Elastostatic force acting on CNT/CNF/nanostructure;F_(Elec)=Electrostatic force; and F_(vdW)=Van der Waals force. Thevoltage source in FIG. 25 may be DC or AC source depending onapplication.

The structure in FIG. 25 may also be used as an electron beam emitterfor use in a display, wherein the position of the nanostructure iscontrolled while electrons are emitted from the structure onto, forinstance, a fluorescent screen that emits photons when excited byelectrons, thus providing a visible point. In this way, a display unit(pixel) with localized geometry control (sub pixels) is provided. Byforming a plurality of these display units into a system of electronbeam emitters, a display for use as a computer screen or televisionapparatus may be provided. Even without using the position control, thenano structure may find applicability as a pixel generating device dueto the small scale of the complete system.

The structure of FIG. 25 may also be used as a chemical sensor.Super-sensitive chemical sensors can be obtained by functionalization:by functionalizing the tip of the free-standing nanostructure it ispossible to attach different kinds of molecules. By actuating the nanostructure by applying a bias (DC/AC depending on requirements) betweentop electrodes and bottom electrode/electrode N, it is possible todetect a molecule that binds to the tip by measuring the current flowthrough it.

Example 2 Control

This example presents results that evidence control over the morphologyand control over the chemical composition present at the base and thetip of grown carbon nanostructures, see FIGS. 26A and 26B. FIG. 26A is atransmission electron microscopy (TEM) micrograph showing a carbonnanofiber grown on a W metal underlayer. FIG. 26A shows how themorphology can differ based on sample preparation recipe.

FIG. 26B shows an example of how the chemical composition at theinterface (base) and at the tip can be obtained. In FIG. 26B panel (a)there is a TEM image of a grown carbon nanofiber; in panel (b) an EDSspectrum shows the chemical elements at the tip of the fibers (catalystregion); and in panel (c) an EDS spectrum shows the chemical elements atthe base of the fibers (underlayer region).

The CNF grew from a flat catalyst surface and no significant catalystfilm break up was observed (see, e.g., Kabir, M. S.; Morjan, R. E.;Nerushev, O. A.; Lundgren, P.; Bengtsson, S.; Enokson, P.; Campbell, E.E. B., Nanotechnology, (4), 458, (2005), incorporated herein byreference).

Example 3 Incorporating Nanostructures into a CMOS Device

Nanostructures as described herein can be incorporated into a CMOSdevice as vertical interconnects. To accomplish this, a filler layersuch as an insulator is deposited over a substrate and thenanostructures situated thereon, and then polished/etched back until thenanostructure is exposed at the top. The catalyst layer can be removed,e.g., by etching, once the nanostructure is grown if required.

Example 4 Lift-Off Method for Growing Localized Nanostructures

The present invention also encompasses a method of making nanostructuresthat are localized at specific positions, rather than being formed inarrays from a continuous film on a substrate. This method obviates therequirement of other processes in the art to anneal a film of catalystto create discrete particles of catalyst in an uncontrolled manner.

According to this method, a metal layer, e.g., on a silicon substrate,is coated with a polymer layer. Such a polymer layer may be aphoto-sensitive layer. The polymer layer is patterned by one of theseveral methods known in the art to define regions where one or morenanostructures are desired. The regions of polymer so patterned, i.e.,where the nanostructures are intended to be positioned, are thenremoved, thus forming cavities in the polymer layer. A layer ofinsulator, e.g., amorphous silicon, is deposited over the polymer,followed by another layer of catalyst. The surrounding polymer layer isthen removed, leaving defined regions such as dots of silicon, withcatalyst on top. Such regions are bases upon which nanostructures canthen be further constructed according to the various methods furtherdescribed herein.

Examples 5-7

In these examples, the results of experiments concerning the PECVDgrowth of nickel-catalyzed free-standing carbon nanotubes on six CMOScompatible metal underlayers (Cr, Ti, Pt, Pd, Mo, and W) are reported.These experiments focus in part on determining the optimum conditionsfor growing vertically aligned carbon nanotubes (VACNTs) on metalsubstrates using DC PECVD. Two sets of experiments were carried out toinvestigate the growth of VACNTs: (i) Ni was deposited directly on metalunderlayers, and (ii) a thin amorphous layer of Si was deposited beforedepositing the Ni catalyst of the same thickness (10 nm). Theintroduction of an amorphous Si layer between the metal electrode andthe catalyst was found to produce improved growth activity in mostcases.

For many electronic applications it is desirable to use a metal whichhas a work function close to that of CNTs, i.e., ˜5 eV, forinterconnects with nanotubes. Metals with work functions ranging from4.33 to 5.64 eV were chosen. In these examples, the result ofinvestigations related to the electrical integrity of the metalelectrode layer after plasma treatment, the quality of the metalunderlayers as interconnects and the quality of the grown CNTs isreported.

Experimental Conditions For Examples 5-7

Oxidized silicon substrates 1 cm² in area and 500 μm thick with an oxide(SiO₂) thickness of 400 nm were used. Cross sections of the preparedsubstrates are shown schematically in FIGS. 27A and 27B. (The relativethicknesses of the layers are not to scale.) First, the metal electrodelayer (for example, Cr, Ti, Pt, Pd, Mo, or W) was evaporated directly onthe substrate by electron beam evaporation to a thickness of 50 nm.Thereafter, either a 10 nm thick Ni film was deposited partiallycovering the underlying metal layer (FIG. 27B), or an intermediate 10 nmthick amorphous silicon layer was deposited prior to the deposition ofthe Ni layer (FIG. 27A). Si and Ni were evaporated at ˜3×10⁻⁷ mbarchamber pressure to avoid the formation of any non-stoichiometricSiO_(x) on the surface.

A DC plasma-enhanced CVD chamber was used to grow the nanotubes on thestructures of FIGS. 27A and 27B. The experimental set-up and detailedgrowth procedure were as described in Morjan, R. E., Maltsev, V.,Nerushev, O. A. and Campbell, E. E. B., Chem. Phys. Lett., 383, 385-90,(2004). The substrate was placed on a 2 cm diameter molybdenum groundedcathode that contains an Ohmic heater. The temperature of the cathodewas measured via a thermocouple connected to a temperature controller.Thermal gradients across the heater body did not exceed a few Kelvin;additional testing without plasma revealed that heat losses from thesurface were reasonably small, and that the substrate temperature waslower than the heater body by 10-15 K. The opposite effect of heatingthe substrate from the plasma sheath is estimated to be negligibly smalldue to the low current density and total power released in the discharge(two orders of magnitude less than used in other work such as: Cassell,A. M., Ye, Q., Cruden, B. A., Li, J., Sarraazin, P. C., Ng, H. T., Han,J., and Meyyappan, M., Nanotechnology, 15, 9, (2004); and Teo, K. B. K.,Chhowalla, M., Amaratunga, G. A. J., Milne, W. I., Pirio, G., Legagneux,P., Wyczisk, F., Pribat, D. and Hasko, D. G., Appl. Phys. Lett., 80,2011-3, (2002)). The nanotube growth was carried out in a C₂H₂:NH₃ (1:5)gaseous mixture at 5 mbar chamber pressure for all of the experimentalruns. The substrate was heated up to the growth temperature of 700° C.under a low vacuum pressure of 0.13 mbar with 3.8° C.s⁻¹ ramping rate.The breakdown voltage applied at the anode for plasma ignition was 1 kV.After introducing the gas mixture in the chamber, the voltage dropped to400V. The current density at the cathode surface was 0.5-1 mA cm⁻². Thegrowth period was 15 minutes for all investigated substrateconfigurations. Note that a desire for accurate temperature controlimposed a limitation on set-up design. The heater body and substrate aregrounded, and the I-V characteristic of the discharge is limited bynormal glow discharge conditions, i.e., the current density is almostconstant and the total power released in the discharge is governed bythe operational pressure. The potential drop between the cathode andanode is inversely proportional to the gas density and depends on theinter-electrode distance and gas composition.

After growth, the samples were cooled down to room temperature beforeair exposure. Films grown in this way were then imaged with a JEOL JSM6301F scanning electron microscope (SEM). Atomic force microscopy (AFM)was also employed to qualitatively study the substrate morphology afterthe different processing steps. All the experiments were repeated toverify their reproducibility.

Example 5 Catalyst Deposited Directly on Metals (No Intermediate SiLayer)

FIG. 28 shows SEM images of the substrates after the growth sequencewhere a layer of nickel catalyst was deposited directly on top of themetal underlayer. In most cases no CNT growth is observed. The lack ofgrowth observed on both Cr and Ti metal underlayers is contrary toprevious work. For example, Ti and Cr have been used before as bufferlayers between the catalyst and the native oxide covering of a siliconsubstrate to avoid the formation of nickel silicides during PECVD growthof carbon nanotubes or nanofibers (see, e.g., Han, J. H., and Kim, H.J., Mater. Sci. Eng. C 16, 65-8, (2001); and Merkulov, V. I., Lowndes,D. H., Wei, Y. Y., and Eres, G., Appl. Phys. Lett., 76, 3555, (2000)).Also, Ti and Cr have been found to be the optimum metal underlayers forplasma-enhanced CVD growth of nanotubes using Ni and Co/Ni catalysts(see, e.g., Cassell, A. M., Ye, Q., Cruden, B. A., Li, J., Sarraazin, P.C., Ng, H. T., Han, J. and Meyyappan, M., Nanotechnology, 15, 9,(2004)). However, the difference between the instant results and thosereported previously may be related to the difference in experimentalconditions. In particular, the Ti and Cr layer was deposited directly onan Si substrate with native oxide in the case of Cassell, A. M., Ye, Q.,Cruden, B. A., Li, J., Sarraazin, P. C., Ng, H. T., Han, J. andMeyyappan, M., Nanotechnology, 15, 9, (2004) and not on a thick layer ofSiO₂ as here.

In the instant example, a much thicker (400 nm) oxide layer was used toprovide a good insulating layer between the silicon and the metalelectrode. The films where Ni has been deposited on Cr and Ti lookrather smooth in the SEM pictures. AFM investigations of the substratesafter heating, without the growth step, show that Ni on Cr and Ti doesindeed produce a smooth surface after heating. Usage of otherunderlayers shows the presence of islands after heating, with averagedimensions of 20-50 nm diameter and 1-5 nm height.

The SEM picture of a Ni film on a Pt underlayer after growth (FIG. 28)panel (c) shows the presence of 20-40 nm islands. This is very similarto the structure of the substrate after heating, which was alsoinvestigated with AFM. No evidence for nanotube formation can be foundin this sample. In contrast, the Ni—Pd combination (FIG. 28, panel (d))leads to the formation of large irregular shaped columns after thegrowth process. In this case some small nanotube-like structures can beseen with diameters below 100 nm but with very low density of surfacecoverage.

AFM topographical images revealed the formation of small particles afterthe heating step in the Ni—Pd sample, though the impact of particleformation is not evident after the growth sequence. Only the Ni/Mo andNi/W combinations (FIG. 28, panels (e) and (f)) lead to the formation ofVACNT's under these growth conditions. The structures all showed goodvertical alignment with the catalyst particle at the tip. The diameterwas rather small, in the range 540 nm, with lengths in the range 0.5-1μm. The density was, however, very low, with values of 5 nanotubes μm⁻²for Ni/Mo and 73 nanotubes μm⁻² for Ni/W. The diameter distribution isplotted in FIG. 29.

Example 6 Effects of an Intermediate Si Layer on the Growth of Nanotubes

Since the first application of PECVD for growth of vertical alignednanotube arrays on Ni films (Ren, Z. F., Huang, Z. P., Xu, J. W., Wang,J. H., Bush, P., Siegal, M. P., and Provencio, P. N., Science, 282,1105-7, (1998), incorporated herein by reference), researchers havediscussed the role of surface morphology, catalyst thickness and etchingreactions at the surface for the formation of catalyst particles.Silicide formation has been considered to be disadvantageous fornanotube growth and metal layers were used to prevent the formation ofsilicides (see, e.g., Han, J. H., and Kim, H. J., Mater. Sci. Eng. C 16,65-8, (2001); and Merkulov, V. I., Lowndes, D. H., Wei, Y. Y. and Eres,G., Appl. Phys. Lett., 76 3555, (2000), both of which references areincorporated herein by reference in their entirety). Recently, thedetailed investigation of catalyst particles found in nanotubes grown onan iron catalyst was performed with energetically filtered TEM (Yao Y.,Falk, L. K. L., Morjan, R. E., Nerushev, O. A. and Campbell, E. E. B.,J. Mater. Sci., 15, 583-94, (2004), incorporated herein by reference).It was shown that the particles contain significant amounts of Si.Similar observations were made for CNTs grown with PECVD on Nicatalysts. Thus, silicides do not poison the nanotube growth and thequestion about the stoichiometry of the most favourable catalyticparticles is still open. The results reported here exploit thesilicidation process for catalyst island formation. By introducing Si asa sandwich layer between the catalyst and the metal underlayer, asignificant improvement in growing nanotubes on different metalunderlayers was achieved. This can clearly be seen in the series of SEMpictures shown in FIG. 30. Very low density growth was found for Ti,(FIG. 30, panel (a)) and no growth for Cr metal (FIG. 30, panel (b))underlayers. In the case of Cr, many cracks and voids were created onthe film after 15 min in the plasma growth chamber. In the case of Ti,nanotubes are seen to grow from some catalyst sites. These appear to berandomly grown nanotubes with diameters ranging from 10 to 50 nm andlengths extending up to several microns. They show no vertical alignmentand there is no evidence for tip growth. VACNTs grew successfully on theother four substrates, however. The samples with Pd (FIG. 30, panel (d))also contained long non-aligned filamentous structures. Although TEMinvestigations have not been performed, the coexistence of those twotypes of carbon nanostructures looks very similar to results obtained byothers (see, e.g., Melechko, A. V., Merkulov, V. I., Lowndes, D. H.,Guillorn, M. A., and Simpson M. L., Chem. Phys. Lett., 356, 527-33,(2002), incorporated herein by reference). Thus, long non-alignedfilaments may be attributed to CNTs grown by the base-growth mode.

The highest density, 390 nanotubes μm⁻², and most uniform samples weregrown on the Ni/Si/Pt layers on FIG. 30, panel (c)), but the averagelength was shorter than that of the Pd and W cases (0.2-1 μm). A longergrowth time leads to longer individual structures. In order to make aquantitative comparison of different samples, a statistical analysis ofthe top-view SEM images was performed. The size distributions of thebright spots on the images are plotted in FIG. 31. Bright spotscorrespond to a top view of catalyst particles on CNT tips. Diameterswere calculated on the basis of the visible area of the spots. A sideview of one of the samples is shown in the insertion, FIG. 32(e). It isclearly visible that even the smallest spots correspond to verticallyaligned nanotubes. The diameter varies from a few nanometers to morethan 100 nm, and the length ranges from 0.2 μm up to 1 μm. Note that thenanotube diameter is slightly larger than the observed catalyst particlesize, which is statistically more important for thinner objects. Themolybdenum underlayer (FIG. 30, panel (f)) showed the lowest density ofthe four successful layers (89 nanotubes μm⁻²) but also the longeststructures (0.5-2 μm). High-resolution SEM studies (a sample is shown inFIG. 32( e)) revealed that in all four cases VACNT growth occurred via atip growth mechanism as evidenced by the presence of the catalystparticles at the tips. Despite this fact, the grown nanotubes differ interms of diameter, density and length.

The particle diameter distribution, FIG. 31, is strongly shifted tosmaller diameters compared to previously published results where a Nicatalyst is deposited directly on the Si substrate (see, e.g. Chhowalla,M., Teo, K. B. K., Ducati, C., Rupesinghe, N. L., Amaratunga, G. A. J.,Ferrari, A. C., Roy, D., Robertson, J. and Milne, W. I., J. Appl. Phys.,90, 5308, (2001); and Meyyappan, M., Delzeit, L., Cassell, A. M. andHash, D., Plasma Sources Sci. Technol., 12, 205, (2003), both of whichare incorporated herein by reference in their entirety). The averagediameter of ˜10 nm n is much smaller than for Ni catalysed VACNT growthreported in previously published articles (see, e.g., Chhowalla, M., etal., J. Appl. Phys., 90, 5308, (2001); Meyyappan, M., et al., PlasmaSources Sci. Technol., 12, 205, (2003); Cassell, A. M., et al.,Nanotechnology, 15, 9, (2004), incorporated herein by reference; andHan, J. H., and Kim, H. J., Mater. Sci. Eng. C 16, 65-8, (2001),incorporated herein by reference). AFM scans were performed after theheating step and showed no significant difference in surface morphologyfor the situations with and without the silicon intermediate layer. Theformation of small catalytic particles is not only related to theheating step but is also related to the etching of these particles byspecies formed in the plasma (Han, J. H., et al., Thin Solid Films, 409,120, (2002); and Choi, J. H., et al., Thin Solid Films, 435, 318,(2003), both of which references are incorporated herein by reference intheir entirety) as well as metal dusting processes induced by the carbondiffused into the catalytic particles (see Emmenegger, C., Bonard,J.-M., Mauron, P., Sudan, P, Lepora, A., Grobety, B., Zuttel, A., andSchlapbach, L., Carbon, 41, 539-47, (2003), incorporated herein byreference).

The size distribution of VACNTs present on the samples preparedaccording to this example, depends on the presence or absence ofamorphous Si as an intermediate layer. In all samples with an amorphousSi intermediate layer, there is a strong inclination towards formingVACNTs with very small diameters. The distribution is plotted on alogarithmic scale in FIG. 33 (panel (a)) for the case where Si was usedas an intermediate layer. More than 50% of the nanotubes have diameters≦5 nm for the case of Pd and W, with the measured population droppingrapidly for larger diameters. Samples with a Pt underlayer have a broaddistribution up to 35-nm diameter accounting for about 60% of allstructures before dropping rapidly. The Mo underlayer produces a higherpercentage of large diameter structures. FIG. 33 (panel (b)) shows thesize distribution for growth on Mo and W underlayers where no Siintermediate layer was present. The probability peaks at 22 nm forgrowth on W with a FWHM of 20 nm. The distribution for the Mo underlayerappears to be rather random, which is clearly seen in the SEM images(see FIG. 30(f)).

Example 7 Electrical Measurements of Carbon Nanotubes

The electrical integrity of the underlying metal electrode layer afterplasma treatment, and the quality of the metal-nanotube contact areimportant issues for application of CNTs in CMOS compatible devices.Three different configurations of electrodes have been used for carryingout two-probe I-V measurements on the films: (i) both probes on themetal layer; (ii) one probe on the metal layer, and one on the nanotubesurface; (iii) both probes on the nanotube surface. FIG. 34 displays themeasurement configurations and equivalent DC circuit diagrams for eachof these embodiments. Probes with a tip diameter around 40-50 μmconnected to an HP 4156B parameter analyzer via a shielded box were usedto carry out the measurements at room temperature. The probes werebrought in contact with the surface (especially for the case of a CNTsurface) with the help of micromanipulators while monitoring the currentflow through the circuit. Thus it was ensured that the probe touchedonly the CNT surface and not the bottom of the film. The measurementswere carried out to get qualitative results, rather than quantitativeinformation about the film and the metal underlayers. Linear I-Vprofiles were measured for the CNT-metal configuration for the Mo and Wunderlayers (inset of FIG. 35 panel (a)) without the intermediate Silayer separating the metal from the Ni catalyst. Linearity in the I-Vplots suggests ohmic contact between the nanotubes and the metalliclayer. No significant conductance variation is observed in this caseamong the three different measurement configurations, which is expectedas the density of the nanostructures is very low. The main part of FIG.35 panel (a) shows plots for samples containing an intermediateamorphous silicon layer. The resistance is higher than for the situationwithout the amorphous silicon, as could be expected. However, the plotsshow predominantly linear behaviour, with slight nonlinearity fortungsten, suggesting varying degrees of ohmic contact between the CNTand the respective underlying metals.

FIG. 35 panel (b) presents the deviations of conductance values from the1/R value for the metal-metal configuration, represented by the dottedline. The dotted line is used to differentiate between surface leakageand poor contacts. The individual conductance values of differentmeasurement configurations for given metal underlayers are evidenced bystraight line indicators. The high conductance of CNT-CNT configurationsfor Pt and Pd is likely to be due to dominant leakage currents throughthe CNT film which appear in conjunction with the relatively high CNTdensity. It may also be related to an increased effective contact probearea due to the presence of long non-aligned CNTs (FIGS. 31(c), (d)). Onthe other hand, the low conductance value of the CNT-metal configurationfor Pt indicates a very poor metal-CNT contact. For W the inclusion ofCNTs in the measurements leads to progressively lower conductancecorresponding to a contact resistance of ˜150 for theprobe-CNT-substrate system. The constant conductance values in all probeconfigurations for the case of Mo are probably due to the low density ofnanostructures present per unit area. Similar results were obtained forNi deposited directly on W and Mo as discussed above. The low surfacedensity of the CNTs leads to an effective probe-metal-probeconfiguration when the electrical measurements are carried out evenafter the CNT growth. Growth of individual vertically aligned carbonnanostructures on prefabricated metal substrates may simplify CNT-baseddevice fabrication processes compared to, for example, technologieswhich involve the use of CNT dispersions followed by assembly andintegration of CNTs into functional forms by AFM manipulation, AC fieldtrapping of CNTs or chemical functionalization. In the present case, thelinearity of the I-V characteristics on the Si inclusion samples provesthat the electrical integrity of the metal electrodes after plasmatreatment remains stable. The values of the conductance for themetal-Si-CNT configuration scale as follows: Pt<Pd<Mo<W according toFIG. 35( b). According to the circuit diagram, the metal-metalconfiguration provides information concerning the resistance of theprobe and the metal underlayers. The metal-CNT configuration providesinformation related to the resistance R₃ and the CNT-CNT configurationprovides information related to any surface leakage induced currentflowing through the circuit. For example, as indicated in the equivalentcircuit diagram (FIG. 34), if R(CNT−CNT)≦(R₃+R_(Metal)+R₃′), surfaceleakage current will dominate, whereas a poor conductance value for themetal-CNT configuration on Pt metal underlayers reveals that theresistance related to R₃ is the dominant factor. Moreover, because ofthe dominant R₃, Pt may not be a good choice for growing verticallyaligned nanotube-based devices. Due to the low R₃ resistance and noR(CNT-CNT) observed, W was found to be the best metal for interconnectsfrom this set of experiments. Mo and Pd electrodes are also goodcandidates for fabricating devices based on CNTs.

Example 8 Influence of Catalyst Particles

An important aspect is the type and morphology of the catalyst particlesfrom which nanostructures grow (see, e.g., Han, J. H., and Kim, H. J.,Mater. Sci. Eng. C 16, 65-8, (2001); and Ducati, C., Alexandrou, I.,Chhowalla, M., Robertson, J., and Amaratunga, G. A. J., J. Appl. Phys.,95, 6387, (2004)). In the instant studies, AFM analyses confirmed theformation of 1-5 nm rough, particle-like structures after the heatingstep for most of the metal underlayers, but not all of them catalyze thegrowth of carbon nanotubes. The formation and dimension of theNi-containing islands is therefore not the only deciding factor for thegrowth of the nanostructures. It has been reported previously that thecatalyst particles are not pure metal, but rather a complexcrystallographic phase containing Ni, and C (Ducati, C., Alexandrou, I.,Chhowalla, M., Robertson, J., and Amaratunga, G. A. J., J. Appl. Phys.,95, 6387, (2004)), and even Si inclusion does not poison iron-basedcatalysts (Yao Y., Falk, L. K. L., Morjan, R. E., Nerushev, O. A. andCampbell, E. E. B., J. Mater. Sci., 15, 583-94, (2004)).

In the instant study, the substrate configuration is changed due to thepresence of a metal underlayer above a thick amorphous silicon oxidelayer, which influences the growth of the individual nanotubes. Chemicalmechanisms that may impact on nanotube growth include intermetallicdiffusion, metal silicidation and gas phase reactions in the CVD chamber(see, e.g., Chhowalla, M., et al., W. I., J. Appl. Phys., 90, 5308,(2001); and Meyyappan, M., et al., Plasma Sources Sci. Technol., 12,205, (2003)). Following the experimental procedure described herein, theprocesses which are responsible for the CNT growth can be divided intotwo main stages: (i) the ramping stage—before the samples reach thegrowth temperature of 700° C. where the metallic interlayer interacts toform different alloys under vacuum conditions, and (ii) the growthstage—after the gaseous mixture is released into the reaction chamberreacting with the catalyst structures formed during the ramping stage.

For the catalyst-silicon-metal system, two interaction sites are ofinterest during the ramping stage: the interface between the Ni film andthe amorphous Si intermediate layer, and the metal/Si interface. Thefinal product (the catalyst for VACNT growth) of these interactions willdepend on competitive processes defined by the various reaction rates.Aboelfotoh et al. (Aboelfotoh, M. O., Tawancy, H. M., and d'Heurle, F.M., Appl. Phys. Lett., 50, 1453, (1997), incorporated herein byreference) used electron diffraction studies to show that the Ni and Siwill not interact at a temperature lower than 120° C., and the mostfavourable compound formed at 600° C. is an amorphous/crystalline phaseof Ni—Si, when 20 nm Ni is deposited on top of a 40 nm amorphous Silayer. Details regarding the Si/metal and metal/SiO₂ interfaces arediscussed below in separate examples.

In the second stage, when the gases are introduced into the reactionchamber and the plasma is ignited, one can expect further modificationsto the surface topography and stoichiometry. In the present version ofPECVD with a DC glow discharge, the plasma conditions in the vicinity ofthe substrate are defined by the gas type, pressure and gas temperature.The modification of these parameters results in different nanostructuregrowth. Similar gas compositions to those employed herein were used byBell et al. (Bell, M. S., Lacerda, R. G., Teo, K. B. K., Rupesinghe, N.L., Amaratunga, G. A. J., Milne, W. I. and Chhowalla, M., Appl. Phys.Lett., 85, 1137, (2004), incorporated herein by reference) who reportedthat the most dominant species in a C₂H₂:NH₃ plasma at 700° C. are theneutral molecules C₂H₂, NH₃, H₂, N₂ and HCN and the positive ions NH³⁺,C₂H₂ ⁺, NH₂ ⁺, NH⁴⁺, HCN⁺ and C₂H⁺. The catalytic decomposition of thesespecies provides carbon atoms and dimers, which will dissolve in thealloyed catalyst producing carbon-based materials. These materialsdepend on the catalyst structure which was formed during the first stageand therefore depend on the metal underlayers. Studies concerning therole of ammonia reveal that its activity is related to removingamorphous carbon and carbon compounds in the supersaturated particles(see, e.g., Han, J. H., et al., Thin Solid Films, 409, 120, (2002); andChoi, J. H., et al., Thin Solid Films, 435, 318, (2003), both of whichare incorporated herein by reference in their entirety). It was alsoreported that hydrogen derived from NH₃ not only assists in thedehydrogenation of the absorbed hydrocarbons, enhances the surfacediffusion of carbon and etches amorphous carbon, but also influences thegrowth morphology (Hash, D. B. and Meyyappan, M., J. Appl. Phys., 93,750, (2003), incorporated herein by reference). This study shows that,for the cases when the growth of VACNTs was not possible, NH₃ can eithercause etching of all the carbon species once they attach to the catalystor leave the carbon species to saturate the catalytic particle formingamorphous carbon and graphitic deposits. Note that partialconcentrations of fragmented and charged species in the gas phase dependon other discharge conditions such as hot filament gas activation.

Example 8 Cr and Ti Metal Underlayers

According to recently published results, Ti and Cr represent the mostpromising metal underlayers with respect to VACNT growth when Ti or Crfilms are deposited on a Si substrate with native oxide (see, e.g.,Cassell, A. M., et al., Nanotechnology, 15, 9, (2004); Han, J. H., andKim, H. J., Mater. Sci. Eng. C 16, 65-8, (2001); and Merkulov, V. I., etal., Appl. Phys. Lett., 76, 3555, (2000), both of which references areincorporated herein by reference in their entirety). The resultspresented herein show the opposite behavior. The contradiction betweenthese results can be attributed to the growth parameters that differ inthe type of supporting layer below the metal layer, presence/absence ofa hot filament system, current density, heating system and temperaturecontrol of the substrate.

The current density in the instant example was fixed by normal dischargecurrent density. It is lower, e.g., in comparison to (Cruden, B. A.,Cassell, A. M., Ye, Q. and Meyyappan, M., J. Appl. Phys., 94, 4070(2003); Teo, K. B. K., et al., Nano Lett., 4, 921-6, (2004), both ofwhich are incorporated herein by reference in their entirety) by twoorders of magnitude. This means that heating effects from the plasma,discussed in detail therein, play a much smaller role in the instantexample, and the relative input of ion flux in comparison with neutralcarbon and etching gases is small. On the other hand, this flux isenough to supply VACNT growth on other metal sub-layers and may not bethe main reason for the lack of VACNT growth. The role of a hot filamentin PECVD of nanotubes was investigated in Cruden, B. A., et al., J.Appl. Phys., 94, 4070 (2003), incorporated herein by reference. It wasshown that the hot filament does not improve the quality of CNTsrelative to conventional DC PECVD. Thus, the difference in plasmaconditions seems not to be the main reason for the contradiction. Othersets of possible reasons for the difference in CNT growth might berelated to the difference in chemical reactions at all interfaces in thesystem-Si (or SiO₂)/Ti, Ti/Ni, Ti/Si. The different substratecomposition (Si versus SiO₂) leads to different chemical reactionsduring the growth process. Cr—Ni and Ti—Ni phase diagrams show thatpossible reactions are likely to occur at 700° C. Reader, et al.(Reader, A. H., van Ommen, A. H., Weijs, P. J. W., Wolters, R. A. M.,and Oostra, D. J., Rep. Prog. Phys., 56, 1397-467, (1993), incorporatedherein by reference) discussed, in their review of silicidationmechanisms, that the reaction between Ti and SiO₂ at 700° C. willconvert Ti into TiSi₂. The oxygen is released upwards from the siliconoxide layer and may inhibit the Ni catalyst film in the given substrateconfiguration.

Moreover, Ti reacts with Si even at room temperature, forming Ti—Sicompounds (Reader, A. H., van Ommen, A. H., Weijs, P. J. W., Wolters, R.A. M., and Oostra, D. J., Rep. Prog. Phys., 56, 1397-467, (1993),incorporated herein by reference), which may explain the inefficiency ofthe Si interlayer inclusion in that case in comparison to the othermetal underlayers. Presumably, the total amount of Si is consumed by theTi underlayer before it reaches the growth temperature, forming aTiSi/TiSi₂ layer (3 min. ramping to reach 700° C.). The Cr metalunderlayer reacts with Si and forms CrSi₂ silicides at 450° C. (Hung, L.S., Mayer, J. W., Pai, C. S., and Lau, S. S., J. Appl. Phys., 58,1527-36, (1995), incorporated herein by reference). The reactionmechanisms between as-formed titanium or chromium silicides and Ni havenot been well-studied. The only information that may be extracted fromthe AFM topographic studies on the heated substrates is the largerparticle formation for both cases (Si inclusion and exclusion) incomparison to other metal underlayers. Apparently for the Cr and Tilayers, the reactions that occur during the ramping stage and the plasmaenvironment stage collectively do not lead to any favorable conditionsto produce carbon nanostructures either with or without an Siintermediate layer. Such a dramatic difference may be related to anearlier start of silicon oxide reduction under the metal followed byoxygen diffusion to the reaction zone. Further particle refinement inthe presence of carbon-containing gases may be suppressed by the excessoxygen, thus hindering the CNT growth. To overcome this effect a highercarbon flux is required, but the instant discharge configuration did notallow increase of the current density or activate the carbon supply.

All the speculation on titanium silicide and oxide formation andinhibition of Ni catalytic activity becomes more important in the caseof non-equilibrium heating obtained in (Meyyappan, M., Delzeit, L.,Cassell, A. M. and Hash, D., Plasma Sources Sci. Technol., 12, 205,(2003); and Cassell, A. M., Ye, Q., Cruden, B. A., Li, J., Sarraazin, P.C., Ng, H. T., Han, J. and Meyyappan, M., Nanotechnology, 15, 9, (2004);Cruden, B. A., Cassell, A. M., Ye, Q. and Meyyappan, M., J. Appl. Phys.,94, 4070 (2003); and Teo, K. B. K., et al., Nano Lett., 4, 921-6,(2004), all of which references are incorporated herein by reference intheir entirety) when a high power plasma discharge is used for the gaspreparation and substrate heating. In this case, the hottest part of thesubstrate is the catalyst layer, and the transformation of catalyst andformation of Ni carbide starts faster than another solid state reaction.Catalyst nanoparticles and CNT embryos have time to start to grow beforethe Ti or Cr underlayers transform to silicides. This speculativeexplanation requires further investigations. A disadvantage of thehigher current density is that it may cause damage of the substrate andgrown nanostructures that will strongly limit the application of themethod for the growth of small CNT bundles or individual structures.

Example 9 Pd and Pt Metal Underlayers

For the case of Pd and Pt, AFM measurements reveal the formation ofsmall particles after the heating step. The phase diagrams show that nopredominant alloy formation is likely to happen between Ni—Pd and Ni—Ptat 700° C. (Massalski, T. B., Binary Alloy Phase Diagrams, vol. 2, Fe—Ruto Zn—Zr (1986, Metals Park, OH: American Society for Metals),incorporated herein by reference). In the present layer configurations,Ni—Si—Pt/Ni—Si—Pd, the first reactions are the transformation of thePd—Si and Pt—Si interfaces to crystalline silicides (Pd₂Si and Pt₂Sirespectively) (Aboelfotoh, M. O., Alessandrini, A. and d'Heurle, M. F.,Appl. Phys. Lett., 49, 1242, (1986); Reader, A. H., van Ommen, A. H.,Weijs, P. J. W., Wolters, R. A. M., and Oostra, D. J., Rep. Prog. Phys.,56, 1397-467, (1993), both of which are incorporated herein by referencein their entirety). Afterwards, at higher temperatures, the top Ni layerwill start to interact with the remaining amorphous Si and most likelywith the Pt/Pd silicides, thereby forming binary/ternary alloys(Kampshoff, E., Wäachli, N. and Kern, K., Surf. Sci., 406, 103, (1998);Edelman, F., Cytermann, C., Brener, R., Eizenberg, M. and Well, R., J.Appl. Phys., 71, 289, (1992); and Franklin, N. R., Wang, Q., Thobler, T.W., Javey, A., Shim, M. and Dai, H., Appl. Phys. Lett., 81, 913, (2002)all of which references are incorporated herein by reference in theirentirety). Thus, there is a strong chemical difference between theexclusion and inclusion of Si for both the Pd and Pt cases. Moreover,the strong reactions that occur, both at the ramping stage and at theplasma environment stage, collectively result in the formation ofnanostructures with small diameters for the Si inclusion case, but nogrowth for the Si exclusion case. The latter case correlates to the badgrowth of CNTs on an Ir underlayer observed in (Cassell, A. M., et al.,Nanotechnology, 15, 9, (2004), incorporated herein by reference).

Example 10 Mo and W Metal Underlayer

Mo—Ni and W—Ni phase diagrams show the formation of Ni-rich alloys attemperatures higher than 700° C. The integrity of the Ni layer depositedon Mo/W is to some extent affected, leading to a very low density ofindividual nanostructures for the Si exclusion case. The lack ofuniformity and low density of nanostructures from these samples agreeswith the observations made by Franklin et al. (Franklin, N. R., Wang,Q., Thobler, T. W., Javey, A., Shim, M. and Dai, H., Appl. Phys. Lett.,81, 913, (2002), incorporated herein by reference) where the presence ofW/Mo electrodes under the catalyst layer inhibited the growth ofnanotubes, but disagrees with previously published results where Mo/Wcompounds are used as catalysts for nanotube growth (Lee, C. J., Lyu, S.C., Kim, H. W., Park, J. W., Jung, H. M., and Park, J., Chem. Phys.Lett., 361, 469, (2002); and Moisala, A., Nasibulin, A. G. andKauppinen, E. I., J. Phys.: Condens. Matter, 15, S3011, (2003), both ofwhich are incorporated herein by reference in their entirety). Mo and Wstart to consume Si at ˜800° C. and ˜950° C. respectively to formsilicides (Aboelfotoh, M. O., Alessandrini, A. and d'Heurle, M. F.,Appl. Phys. Lett., 49, 1242, (1986); and Murarka, S. P., J. Vac. Sci.Technol., 17, 775, (1980), both of which references are incorporatedherein by reference in their entirety). At present, the investigatedprocesses are below these temperatures. Thus by introducing an Siinterlayer a stable Si—Mo and Si—W system was achieved to facilitate apure Si—Ni surface which apparently enhanced the density of individualnanostructures in the film. Moreover, these metals form a barrier for Siand Ni diffusion in both directions and limit the amount of Si that canreact with Ni in comparison to the case where the Ni film is depositeddirectly on bulk silicon with a native oxide layer.

The effect of the Si interlayer may be compared with experiments on bulkSi having a native oxide layer (˜1 nm), which were also carried out inthe same set-up and under similar conditions. By comparing the catalystparticle/nanotube density (117/75 counts μm⁻²) for growth on an Ni film(10 nm) deposited on silicon substrates with an Si amorphous interlayer(10 nm) between the metal and the catalyst, it was observed that thedensity of nanostructures is increased by a factor of ˜5, 3, 2, 1 forthe Pt, Pd, W and Mo cases respectively. Thus, by tuning the thicknessof the amorphous Si interlayer, one can control the density and particledistribution by changing the stoichiometry of the catalytic particles.

In summary, nanotubes have been successfully grown on four out of sixchosen CMOS compatible metal underlayers by using silicon as anintermediate layer. An important observation from this set ofexperiments is that the size of the nickel islands formed after theheating sequence is not the only deciding factor for nanotube growth.Consequently, these experiments show that Si plays a vital role in thegrowth of carbon nanotubes. Moreover, the Si layer thickness is anadditional tool for tuning the growth of carbon nanotubes with goodquality and quantity as required for a particular application, alongwith the growth temperature, chamber pressure and different gas ratios.In particular, the insertion of a Si layer produces individualvertically aligned nanotubes with small diameter (≦10 nm) which can beadvantageous for many applications.

The studies reported herein showed a poor growth of nanostructures on Tiand Cr metal underlayers, which is in apparent contradiction with theresults obtained by other laboratories. The main reason for such adifference is attributed to Ti silicidation on the thick silicon oxidelayer with a high release of oxygen that influences the Ni/Ti interface.

As metal interconnects, a W underlayer was found to be the bestunderlayer metal for the production conditions described herein.Nevertheless, structural and electrical integrity seems to remain intactfor all the metal underlayers even after the harsh chemical and plasmatreatment.

Example 11 Effects of Silicidation on the Growth of Individual FreeStanding Carbon Nanofibers

This example addresses vertically free standing carbonnanotubes/nanofibers and their integration into functional nanodevices.In this example, growth of individual free-standing carbon nanofibers onpre fabricated catalyst dots on tungsten and molybdenum metalunderlayers are shown, exploiting an amorphous silicon layer as part ofthe catalyst layer. In summary, more than 95% of the catalyst dotsfacilitated nucleation for growth on the W metal underlayer.Silicidation occurring during the growth sequence is suggested to play avital role for growth kinetics. EDX chemical analysis revealed that thetip of the nanofibers consists of an alloy of Ni and an underlayer metaland the base shows the signature of Ni, Si and underlayer metal.

The growth conditions and growth kinetics on different metal underlayersdiffer substantially from the growth mechanism that is postulated for Sisubstrates. This example provides an explanation for the growth resultson W and Mo in terms of silicide formation. Individual nanofibers werecharacterized in a transmission electron microscope (TEM). The elementalcompositions were determined by fine probe energy dispersive X-rayspectroscopy (EDX).

Oxidized silicon substrates 1 cm² in area with an oxide thickness of 400nm were used. First the metal (W or Mo) underlayer was evaporateddirectly onto the substrate by electron beam evaporation to a thicknessof 50 nm. Stripes and dots (100 nm and 50 nm edge to edge distance) werefabricated by e-beam lithography. Experimental details are furtherdescribed in Kabir, et al., Nanotechnology, 17, 790-794, (2006),incorporated herein by reference. An intermediate 10 nm thick amorphoussilicon layer covered by 10 nm of Ni was used to catalyze growth. A DCPECVD chamber was used to grow the nanostructures. The experimentalset-up and detailed growth procedure have been described in Morjan, R.E., et al., Chemical Physics Letters, 383, 385, (2004), incorporatedherein by reference. The nanotube growth was carried out in a gaseousC₂H₂:NH₃ (1:5) mixture at 5 mbar chamber pressure at 700° C. for 20minutes for all of the experimental runs discussed here. The substrateswere first heated up to 700° C. under low vacuum conditions (0.13 mbar)with a 3.8° C./second ramping rate (heating stage). After growth, thesamples were cooled down to room temperature before air exposure.As-grown nanotubes from pre-fabricated dots were then imaged with a JEOLJSM 6301F scanning electron microscope (SEM) or a JEOL ULTRA 55 SEM.Samples were then gently rubbed onto a TEM grid to transfer the grownfibers from the substrate to the grid. Individual fibers were theninvestigated by TEM and EDX.

Morphology changes of the patterned substrate/catalyst layer may occurduring the heating step of the growth sequence, but no predominantcatalyst breakup or cluster formation was observed, which is in goodagreement with experiments in which catalyst films were used. FIG. 36shows SEM images of the substrates after the growth sequence for thecase of W. FIG. 36 panels (a), (c) and (d) show the micrograph of growncarbon nanofibers (CNF) from patterned 100 nm side length dots with 500nm pitch, 50 nm length with 1 μm pitch, and 50 nm length with 500 nmpitch, respectively. As can be seen, more than 95% of the catalyst dotsnucleated for growth. The catalyst from 100 nm dots splits, and multipleCNFs up to 4 fibers per dots were observed. CNFs grown from 50 mm dotsare individual and vertically well aligned. There are some instances ofmultiple CNFs growing from a single dot (˜2%). All cases wherenanofibers grew showed a tip growth mechanism as evidenced by thepresence of the catalyst particles at the tips. No predominant pitchinduced effects are evident for 1 μm and 500 nm pitch respectively.Since an amorphous Si layer is included as a part of the catalyst layeron top of metal underlayers, the interactions between the amorphous Siand the two metal layers (silicidation) are important processes fordefining the final phase of the catalyst and its catalytic activity. Anexample is shown in FIG. 37 panel (b) where only Ni was deposited on W,resulting in no catalytic activity and no growth.

It is reported that at room temperature the stress present in thedeposited film is due to the mismatch in thermal expansion coefficientsbut at elevated temperature silicidation occurs resulting in net volumeshrinkage. The volume decrease can be very large and this could lead tolarge tensile stresses in the silicided films. After heating the tensilestress for Ni and Mo silicides is found to be ˜0.25×10⁻⁹ dyne/cm² and˜0.10×10⁻⁹ dyne/cm² respectively, which are of the same order. Thisperhaps explains why no catalysts broke up during the heating process;the break up into smaller patches is controlled by the growth kineticsrather than induced by the film stress (see inset of FIG. 36( a)).

Silicides can be formed at elevated temperatures either by a solid statereaction between a metal and silicon deposited on each other, or bycodepositing metal and Si. Transition metal silicides have beenextensively studied and explored due to their usefulness as hightemperature materials. The investigated metal underlayers and the Nicatalyst layer should undergo silicidation during nanofiber growth inthis case. For commonly used silicides, when a thin film of metal Mreacts with a thick Si layer the thermodynamically stable phase is MSi₂.Conversely, when a thin Si film reacts with a thick metal layer, athermodynamically stable metal-rich phase is formed. When a thin metalfilm reacts with a thin Si layer where there is neither excess metal norexcess Si present, the equilibrium phase will be determined by the ratioof metal atoms to Si atoms. For a ternary system as described herein,the situation is complicated since two or more phases are likely tooccur simultaneously. In this case the interface reactions anddiffusivities will define the stable phase.

For W—Si and Mo—Si systems, Si is the predominant diffusing species forthe formation of corresponding silicides. On the contrary, Ni is themetal diffusion species in Si at elevated temperatures. All movingspecies are thus presumed to be moving down towards the substrate inthis system. The ramp rate at which the temperature of the substratereaches the growth temperature might also play a role in defining thechemical phase of the silicides. An extensive study on the reaction ofSi with W performed by Nishikawa et al. (Nishikawa, O.; Tsunashima, Y.;Nomura, E.; Horie, S.; Wada, M.; Shibata, M.; Yoshimura, T.; Uemori, R.,Journal of Vacuum Science & Technology B (Microelectronics Processingand Phenomena) (1983), 1, (I), 6) and Tsong et al. (Tsong, T. T.; Wang,S. C.; Liu, F. H.; Cheng, H.; Ahmad, M., Journal of Vacuum Science &Technology B (Microelectronics Processing and Phenomena) (1983), 1, (4),915, both of which are incorporated herein by reference in theirentirety) by field ion microscopy, revealed that Si deposition on W islikely to result in the tetragonal polycrystalline WSi₂ structure at˜700° C., which is also the temperature used herein. However, Tsong etal. reported that a change of silicide phase occurs if heating isextended beyond ˜30 s.

When silicon is the dominant diffusing species, it can continue todiffuse in at a location well beneath the Mo/W interface thus formingsilicides at a distance from the interface. Thus at least two binarylayers: Ni—Mo/W, and Si—Mo/W can be expected to form. It can besuggested that a Si—Mo/W layer provides a platform for the Ni rich Wlayer (Ni—W layer) to catalyze and facilitate CNF growth; no growth isobserved for the case when Ni was deposited directly on W as shown inFIG. 36( b). To support this hypothesis, a TEM investigation on thenanofibers grown on W metal underlayers was carried out as depicted inthe FIG. 26B. FIG. 26B panel (a) represents the typical structure of aCNF from a patterned catalyst of ˜30 nm diameter. The catalyst Niparticle at the CNF tip usually had a conical shape. EDX point analysiswas carried out both at the tip of the CNF and at its base as shown inthe FIG. 26B panels (b) and (c) respectively. The EDX spectra reveal nocharacteristic peak representing Si at the tip of the fibers (FIG. 26Bpanel (b)). W was found to coexist with Ni catalyst at the tip. Howevera small amount of Si is detected at the base of the fibers (FIG. 26Bpanel (c)). Presence of silicon in the catalyst particles (both at thetip and at the base) regardless of catalyst particle type (Ni/Fecatalysts on an Si substrate) is reported by cross sectional TEMobservations. It can be extrapolated from these observations that theparticle at the tip of the CNF was part of the metallurgical layer fromwhich the CNF grew and since in the sample the content of only Ni and Wbut no Si at the tip was observed, it can be surmised that themetallurgical layer for growth in this case was a Ni—W system. It istherefore proposed that a W-silicide layer has provided means for theNi—W layer to nucleate for growth. In the model for tip growth suggestedby Melechko et al., (Melechko, A. V.; Merkulov, V. I.; Lowndes, D. H.;Guillorn, M. A.; Simpson, M. L., Chem. Phys. Lett., 2002, 356, (5-6),527, incorporated herein by reference) the interface between catalyticparticle and substrate is important. By having a silicide rather than apure metal interfacing the catalytic Ni—W particle, these crucialinterface conditions would be altered significantly—apparently in favourof CNF growth. The Mo metal underlayer behaves the same as the W metalunderlayer in many ways; producing CNF with almost the same statisticsin terms of diameter, length, growth yield etc. Mo also behaves similarto W with regards to silicidation. It is therefore proposed that theexplanation regarding the W metal underlayer is valid for Mo as well.

In conclusion, results on CNF PECVD growth have been presented in termsof metal-Si-metal reactions, silicide phases and kinetics. Silicidationis likely to play a vital role in defining the growth mechanism ofnanostructures, where a silicide can enable the upper metallurgicallayer to nucleate. EDX analysis supports this conclusion for the case ofa Ni on Si on W system. Breaking up of the catalyst particles is foundto be more related to growth kinetics rather than the thermal expansioncoefficient of different metals. The silicidation processes for thinfilm metal-Si-metal systems are complex and involve more than onemechanism governing their kinetics.

Example 12 Controlling Nanostructures

This example describes control of CNT/CNF diameter and lengthdistribution in PECVD growth from a single geometrical design. Resultswere obtained by controlling the diameter of catalyst dots by the shotmodulation technique of electron beam lithography. The method comprisesfabrication of dots of different sizes from one single geometricaldesign and the consequent effects on growth of vertically aligned carbonnanofibers on different metal underlayers. Statistical analysis wasundertaken to evaluate the uniformity of the grown CNF structures by thePECVD system, and to examine the achievable uniformity in terms ofdiameter and length distributions as a function of different metalunderlayers. It is possible to control the variation of diameter ofgrown nanofibers to a precision of 2±1 nm, and the results arestatistically predictable. The developed technology is suitable forfabricating carbon based nano-electro mechanical structures (NEMS).

The electrical characteristics (I-V) and switching dynamics of thefabricated devices depend on a number of design and fabrication relatedparameters. Since the CNF/CNT is the active part of the device, both thediameter and the length of the CNTs/CNFs are of great importance. Devicegeometry is depicted in FIG. 25, which shows an electron microscopyimage of a fabricated vertical “nanorelay” structure where theparameters that influences the device characteristics are shown. Asingle CNF is grown between two drain electrodes. The drains areseparated from the source electrode by 400 nm thick SiO₂ insulator.Charge can be induced into the CNF by applying a voltage to the drainelectrode to actuate the CNF. For such two terminal devices, the pull-involtage is defined by the balance of the elastic, electrostatic and thevan der Waals forces (Dequesnes, M.; Rotkin, S. V.; Aluru, N. R.,Nanotechnology, 13(1), 120, (2002), incorporated herein by reference).Since all these three forces are strongly correlated with the diameterand the length of the grown structures and these are the parameters thatcan be controlled experimentally to a certain extent. In this example,is described (a) development of a technology to vary the diameter of theCNFs from one single geometrical design with a precision of 2±1 nm; (b)growing the CNFs on different metal underlayers to realize the optimumthe CMOS platform for CNFs growth; (c) statistical spread and controlover length distribution of the grown structures; and (d) pitchlimitations for mass production of high density parallel structures.

Sample Preparation and Characterization

To fabricate the catalysts dots, the shot modulation technique ofelectron beam lithography is used to define the catalyst dimensions. Theshot modulation technique is a robust technique that has been used forfabricating different kinds of nano-structures. For example, by varyingthe dose applied during the exposure of the two electrode regions, thewidth of the gap between them can be controlled with nanometer precision(see, e.g., Liu, K.; Avouris, P.; Bucchignano, J.; Martel, R.; Sun, S.;Michl, J., Applied Physics Letters, 80(5), 865, (2002), incorporatedherein by reference). The experiment described in this example uses thestate of the art electron beam lithography system, the JBX-9300FS model.The system is capable of keeping the spot size down to ˜6 nm at 500 pAprobe current at 100 kV operating voltage. The system has a heightdetection module which is used to ensure the accuracy of the focus pointof the e-beam spot on the entire work piece and compensate for theheight variation of the resists that usually occurs during spin coatingof the resists.

Oxidized silicon substrates 1 cm² area with an oxide thickness of 400nm, were used. First the metal (=Mo, Nb, or W) electrode layer wasevaporated directly on the substrate by electron beam evaporation to athickness of 50 nm. Sheet resistance measurements were carried out onthe deposited films. Double layer resists system, consisting of 10%co-polymer and 2% PMMA resists, were then spin coated and bakedrespectively. The shot modulation experiments were then carried out oninitial dots of 10×10 arrays with 50 nm square geometry. The same blockwas then distributed in an array of 8×8 matrix and the dose of electronbeam was varied linearly with an interval of 100 μC/cm² starting from500 μC/cm². No proximity corrections were made for dose compensation.Inside the matrix, the columns represent the same dose while the rowsrepresent different doses. The samples were exposed and then developedin a standard developer, IPA:H₂O (93:7) for 3 min.

The samples were then mounted in an e-beam evaporator, and anintermediate 10 nm thick amorphous silicon layer was deposited prior todeposition of the Ni catalyst layer. After the e-beam evaporation, liftoff processes were carried out in Acetone at 60° C., then IPA, andcompleting the sequence by rinsing in DI water and N₂ blow drying.

A DC plasma-enhanced CVD chamber was used to grow the nanostructures.The experimental set-up and detailed growth procedure have beendescribed previously (see, e.g., Morjan, R. E.; Maltsev, V.; Nerushev,O.; Yao, Y.; Falk, L. K. L.; Campbell, E. E. B., Chemical PhysicsLetters, 383(3-4), 385, (2004), incorporated herein by reference). Thenanotube growth was carried out in a C₂H₂:NH₃ gaseous (1:5) mixture at 5mbar chamber pressure at 700° C. for 20 minutes for all of theexperimental runs. The substrates were first heated up to 700° C. underlow vacuum conditions (0.13 mbar) with a 3.8° C. s⁻¹ ramping rate(annealing stage). Once the final temperature was reached, the C₂H₂:NH₃gas mixture was introduced into the chamber and 1 kV was applied to theanode to induce plasma ignition. After growth, the samples were cooleddown to room temperature before air exposure. Nanotubes grown in thisway from pre-fabricated dots were then imaged with a JEOL JSM 6301Fscanning electron microscope (SEM) and JEOL ULTRA 55 SEM. All theexperiments were performed repeatedly to verify their reproducibility.

After each step of the experimental sequences, samples werecharacterized by SEM, as portrayed in FIG. 38. FIG. 38(a) represents the10×10 array of fabricated dots prior to the heating step for growth. Ascan be seen from the figure, the square geometry rounded up to dots.FIG. 38(b) was taken after the heating step prior to exposing the sampleto plasma and gas mixture for growth. Not much seem to happen during theheating step and squared dots remain intact. FIG. 38(c) depicts theresults obtained after the growth sequence. The growth yields more than98% at the dose scale of 1200 μC/cm². Predominant vertical growth ofCNFs was observed. However, for some instances, slight angular deviationfrom perpendicularity of the grown structures was also observed. Inorder to differentiate the impact of the insertion of a layer ofamorphous Si as part of catalyst, a set of experiments in which only Nicatalyst was deposited on W substrates was carried out. As can be seenfrom FIG. 38(d), no growth of CNF is evident. Such results are alsoreported in (Kabir, M. S.; Morjan, R. E.; Nerushev, O. A.; Lundgren, P.;Bengtsson, S.; Enokson, P.; Campbell, E. E. B., Nanotechnology, 16(4),458, (2005), incorporated herein by reference).

Correlation Between Shot Modulation and Catalyst Dimension

The effect of shot modulation on defining the catalyst dimensions,demonstrates the possibility of controlling the diameter of CNF's withnanometer precision. Experiments were carried out on a geometricaldesign set to 50 nm square. All of the metal underlayers gavereproducible results. The electron beam exposure was carried out at 500pA, 100 kV and thereby the beam step size was set to equal a spot sizeof ˜6 nm. FIG. 39 describes the catalyst diameter after metalevaporation as a function of irradiated electron dose during theexposure. The dose was varied by varying the dwell time of the beam oneach exposure shot. Linear increment of the catalyst diameter as afunction of electron dose is expected as the dose was varied linearlyranging from 500 μC/cm² to 1200 μC/cm². For the tungsten layer, below athreshold of 800 μC/cm² electron dose, no catalyst structure wasobserved. The observation can be explained in terms of how the electronenergy is transferred to the resists. During an exposure, a series ofelastic and inelastic scattering events determine the volume over whichenergy is deposited and the resist exposed. When the feature sizes aresmall, this effect becomes even more crucial to define the final exposedpattern. On the other hand, the energy deposited to the resists can bevaried simply by keeping the beam ‘on’ the spot for a longer period.However, in addition to the beam induced parameters, the end outcome ofthe fabricated structures is determined by experimental parameters likeresists thickness, resist developer, solid angle of the metalevaporation, etc. Still, there exist a minimum threshold point belowwhich not enough energy will be transferred to the resists to bedeveloped in the resist developer and no metal structure appears aftermetal deposition and lift off process. This is what is observed in FIG.39. No structure appeared below 800 μC/cm² electron dose. Additionally,this threshold point depends not only on the type of the resists itselfbut also on other parameters such as substrate material, beam currentdensity, beam pitch, etc. Nevertheless, the electron beam lithographytechnique not only facilitated extremely high positional precisioncapability (≦50 nm) but also proved to be a robust technique to controlthe diameter from a single design.

Growth on Different Metal Underlayers

FIGS. 40 and 41 show an SEM of nanotubes grown from catalyst dots ondifferent metal underlayers fabricated at a dose of 800 μC/cm² and 1200μC/cm² respectively, for two different pitches (500 nm and 1 μm) in eachcase. Doses below 800 μC/cm² did not give any growth of CNFs, a factwhich correlates well with the observation of lack of catalyst particlesafter lithography under these conditions (see FIG. 39). The structuresof the grown CNFs were very similar for the Mo and W metal underlayersexcept for the fact that the W metal underlayers required a slightlyhigher dosage to reach the same yield. For the case of tungsten, at thedose of 800 μC/cm², CNFs grew from more than 60% of the total catalystdots. At even higher doses, more than 97% catalyst dots act asnucleation sites for growth of nanotubes. CNFs grew from supportedcatalyst particles via a tip-growth mechanism in the followedconditions. The block with 500 nm pitch, on the other hand, yielded morethan 85% growth from catalyst cites produced at 800 μC/cm². Thisincidence correlates with the proximity effect of the electron dose, andresulted in higher energy deposited to the resists during theprocessing.

Mo and W provided a stable platform for Si—Ni to interact, formingsilicides at the growth temperature without breaking into littledroplets. This result is different from the observations by Yudasaka etal. (see Yudasaka, M.; Kikuchi, R.; Ohki, Y.; Ota, E.; Yoshimura, S.;Applied Physics Letters, 70(14), 1817, (1997), incorporated herein byreference), Merkulov et al. (see Merkulov, V. I.; Lowndes, D. H.; Wei,Y. Y.; Eres, G.; Voelkl, E. Applied Physics Letters, 76(24), 3555,(2000), incorporated herein by reference) and Teo et al. (see Teo, K. B.K., et al., Nanotechnology, 14(2), 204, (2003), incorporated herein byreference) where, for initially large dots, multiple droplets wereformed. As the size of the dots is reduced, the number of Ni dropletsalso decreases. Merkulov et al. observed ˜300 nm critical diameter andTeo et al. observed ˜100 nm critical diameter below which single VACNFsare grown. In all cases, only Ni was used as catalyst layer. Inaddition, in their case, formation of droplets was the necessaryprecursor for the catalytic growth of nanofibers. On the contrary, nodroplet formation is observed after the heating step (see FIG. 38(b)).Similar behaviour was observed even for the case where films of catalystwere used (Kabir, M. S.; Morjan, R. E.; Nerushev, O. A.; Lundgren, P.;Bengtsson, S.; Enokson, P.; Campbell, E. E. B. Nanotechnology, 16(4),458, (2005), incorporated herein by reference). Therefore, theseobservations suggest that the formation of droplets may not be the onlycriterion for catalyst nucleation.

The binary phase diagram of Nb—Si indicates that no reaction shouldoccur at the growth temperature used in the experiment (see, e.g., Zhao,J. C., Jackson, M. R., and Peluso, L. A., Mater. Sci. Eng. A, 372, 21,(2004), incorporated herein by reference). Therefore, a Nb metalunderlayer is also expected to facilitate a stable platform for Si andNi to interact. The silicide formation step is therefore not expected tobe the reason for the poor growth results on the Nb metal underlayer.There are a number of parameters that would influence the growth resultsincluding details of how the metal underlayer and the catalyst layersare deposited.

Furthermore, a Si layer is present between the Ni catalyst and the metalunderlayers. Ni undergoes chemical reactions with Si at growthtemperature 750° C. and forms mono/di silicidates (Kabir, M. S.; Morjan,R. E.; Nerushev, O. A.; Lundgren, P.; Bengtsson, S.; Enokson, P.;Campbell, E. E. B. Nanotechnology, 16(4), 458, (2005), incorporatedherein by reference) and remains stable. The observation may alsoperhaps be due to the fact that below a critical dot size (in this case˜50 nm has rather small volume) the breakup does not occur due toincrease in the surface energy, which is larger than the reduction ofstrain energy imposed by the mismatch of thermal expansion coefficientof different metal layers at a given temperature. Nevertheless, alterthe acetylene is introduced, the VACNF growth begins. Growth mechanismsfollow the tip growth model as is evident from the bright spot at thetip of nanotubes. Only rarely has formation of multiple CNFs from singledots been observed. Since the occurrence of such multiples of CNFs wasless than 3%, the phenomenon is considered to be negligible and remainsto be explained.

Statistical Evaluation

All experiments were performed on 72 blocks of 10×10 arrays of catalystdots for each electron dose. To evaluate the structural uniformity,especially the tip diameter and the height distribution of the grown CNFstructures, statistical analysis was undertaken. The statisticaldistribution was carried out on 75 randomly chosen CNFs for eachelectron dose. The results from statistical distributions are summarizedin FIG. 42 and FIG. 43. FIG. 42 represents the grown CNF tip diameter asa function of catalyst dimension. Standard deviations of the measureddata are shown as error bars for obtained mean values. For instance, theobtained mean value for the tip diameter of the grown CNFs is 26 nm (Wsubstrate) from ˜48 nm diameter catalyst with a standard deviation of±3.5 nm. FIG. 42 also represents a benchmark to predict the results witha statistical accuracy of ±3 nm, which is sufficiently good data tofabricate NEMS structures with statistically predictable I-Vcharacteristics. Moreover, almost linear dependence of the tip diameteron the size of catalyst dimension, which is again dependent on thedeposited electron dose of the EBL, proves to be a robust technique tocontrol the tip diameter with an accuracy of ±2 nm.

As evident from the figures, diameters of the grown CNFs are roughly 50%smaller than the initial catalyst size. This observation is consistentwith others (see Teo, K. B. K., et al., Nanotechnology, 14(2), 204,(2003), incorporated herein by reference). According to the sphericalnanocluster assumption (Teo, K. B. K., et al., Nanotechnology, 14(2),204, (2003), incorporated herein by reference), it is possible tocalculate the expected diameter of the grown CNF by equating thepatterned catalyst with the volume of a sphere. The calculated diametersare thus plotted in dotted lines. The theoretical plot gave very goodagreement with the average experimental values for diameters when thecritical thickness for the catalyst was set to 4 nm. This is 60%reduction from the initial thickness of the catalyst film (initial 10 nmthick Ni catalyst). Moreover, this observation fortifies the fact thatthe silicidation occurs during the growth process, and dominates andcontrols the exact thickness of the catalytically active film.Statistical analysis on length distributions of the grown CNFs showedGaussian distributions for all cases. The most pragmatic parameter fromthe distributions, the FWHM of length distribution, is plotted as afunction of catalyst dimensions in FIG. 43. The spread of the Gaussianfit is also indicated by bar on each point. It is apparent from theFigure that height distributions for W and Mo almost overlap with eachother. Whereas Ni produced more than half the height compare to othermetals. This difference for different metals underlayers suggest thatthe different metals give rise to different pace to the catalyticactivities of the catalysts resulting different length distributions.Moreover, the spread of length distribution is of the order of 100 nmwhich is substantially better than the reported value by others (seeTeo, K. B. K., et al., Nanotechnology, 14(2), 204, (2003), incorporatedherein by reference) where spreads of the order of microns werereported. The height variations as a function of catalyst diameter showa predominantly straight line, which is not surprising as the volume ofthe catalyst does not increase significantly as a function of catalystdimension to produce significant impact on height.

Diameter and Length Distributions

All experiments were performed on 72 blocks of 10×10 arrays of catalystdots for each electron dose (7200 dots for each dose condition). The tipdiameter and nanofiber length were determined for at least 50 randomlychosen structures for each electron dose. The results are summarized inFIGS. 42 and 43.

The length of grown nanotubes ranged from 800 nm to 900 nm. The tipdiameter was ranging from 20 nm to 70 nm. Only a few nanotubes did notgrow normal to the substrate. The grown fibers tend to have largerdiameter at the bottom and smaller at the top, thereby forming conicshape nanofiber structures with conical angle less than 2°. Apparently,e-field alignment is related to number of CNTs growing from each dot.When examining the critical size for the nucleation of single CNFs, itwas discovered that there were still some instances of multiple (i.e.,double) CNFs from some catalyst dots (below 3%). Mo substrate producedbetter yield (more than 80%) at the same electron dose. Structuralconfigurations of the grown structures did not seem to differ between Moand W metal underlayers except where the W metal underlayers requiredlittle higher dosage to reach the same yield. This could be related tothe conductivity of the metal substrates. Nb was chosen as an exoticmaterial simply for the purpose of a comparative analysis with the othermetals. At dose 800 μC/cm², not more than 30% dots nucleated for growth,but this trend remains the same at higher dosage.

FIG. 42 shows the CNF average tip diameter as a function of catalystdimension (i.e., electron dose). The error bars represent standarddeviations in nanometers. An almost linear dependence of the tipdiameter on the catalyst size is observed. Since the catalyst size canbe controlled by adjusting the electron dose in the EBL, this proves tobe a robust technique to control the tip diameter from a single designgeometry with an average standard deviation of ±4 nm. As is evident fromFIG. 42, the diameters of the grown CNFs are roughly 50% smaller thanthe initial catalyst size. The base diameter is slightly smaller thanthe diameter of the catalyst with an average value ranging from 40 to 50nm as a function of dose, i.e., approximately 1.5 times larger diameterthan at the tip (corresponding to a conical angle of about 0.5° for 1 μmlong fibres). This observation is consistent with related studies wherecarbon nanofibers were grown on Ni catalysts of 100 nm dimensions andlarger deposited on a doped silicon substrate with an 8 nm thick oxidebarrier where the measured tip diameters were about 0.5 of metalcatalyst diameter (Teo K B K, et al., Nanotechnology, 14, 204, (2003),incorporated herein by reference). That earlier work was more focused onlarge diameter structures (larger than 100 nm). The measured standarddeviations were smaller than in the instant case; however, this is morerelated to the lithographic challenges of producing small<100 nmstructures than to the growth process. In this case the catalyst tip ofthe grown CNF takes an approximately conical shape (Yao Y, Falk L K L,Morjan R E, Nerushev O A and Campbell E E B, J. Microsc., 219, 69-75,(2005), incorporated herein by reference) and therefore the volume ofcatalyst material enclosed within the CNF tip can easily be estimated.From TEM studies it is possible to estimate the height of the cone to beapproximately 40 nm for a 25 nm diameter CNF. The estimated catalystvolume then turns out to be approximately one-fifth of the originallydeposited catalyst dot volume. The remaining catalyst material ispresent at the base of the CNF in the form of small Ni particlesembedded in a carbon ‘dome’ or in a thin layer of Ni between the carbon‘dome’ and the amorphous silica layer coating the silicon wafer (Yao Y,et al., J. Microsc., 219, 69-75, (2005), incorporated herein byreference).

The measured lengths of the grown CNFs showed Gaussian distributions forall cases. The average length is plotted as a function of catalystdimension in FIG. 43. The standard deviation is indicated by the bar oneach point. It is apparent from the figure that the height distributionsfor W and Mo almost overlap with each other. On the other hand, thenanofibers grown on the Nb underlayer were only slightly more than halfthe height of the fibres grown on the other metals. The spread of thelength distribution for W and Mo metal underlayers varied from 8 to 15%with an average standard deviation of 11%. In contrast, for the Nb metalunderlayer it varied up to 20% with an average standard deviation of16%. There is no dependence of the height of the structures on thecatalyst diameter within the range that has been investigated asdescribed herein.

Pitch Limitations:

The ultimate limit for integrating such densely populated structures wasconsidered. Experiments were carried out on pitches with 1 μm, 500 nmand 100 nm distances to evaluate the pitch induced effects. Nosignificant variations were observed between the 1 μm and 500 nm pitchin terms of height and diameter distributions. However, 50 nm dots with100 nm pitch revealed characteristics that resembled growth from film ofcatalyst. FIG. 44 depicts the results obtained. FIG. 44(a) representsthe catalysts after the heating step where neither break-up of the dotsnor predominant clusters formation (catalysts conglomeration) areobserved. On the other hand, CNFs tend to come close in contact with thenext neighbors while growing. This effect is perhaps due to the localfield gradient that occurs on the CNFs during the growth period. Thepitch limit on the reliable production of individual carbon nanofibersgrown from 50 nm catalyst dots is therefore seen to lie between 100 and500 nm.

In conclusion, use of the shot modulation technique to define thecatalyst diameter exceptionally precisely, thereby controlling the CNFsdiameter and length, has been demonstrated. As predicted by theoreticalmodels, the switching dynamics are sensitive to geometrical parameters.This example demonstrates experimentally that it is possible to tunethese two important parameters related to fabrication of C-NEMSstructures, diameter and length, up to the limit of ±2 nm for diameter,and a spread of ±50 nm for length.

Two CMOS compatible metals, Mo and W, as metal underlayers substantiatedto produce good quality CNFs suitable for C-NEMS device fabrication.These metals to produce low ohmic contacts with CNFs. Therefore they areexpected to create good low ohmic contact with the grown CNF structures,which is essential for device fabrication. On the other hand, Nb did notproduce high quality and reliable results and thereby is preferably notused as source metal underlayer. The developed technology appears topossess a limit of 100 nm pitch for large scale integration.

The results obtained from arrays of same structures on entire ˜1 cm areawere found to be reproducible for each electron dose scale. Thesepositive results also indicate the potential of the technique to be usedfor wafer level integration of the CNFs based NEMS structures. In termsof number, length and diameter, control over the process is sufficientto carry out large scale production of carbon based NEMS structures withpredictable statistical variations. The technology will facilitate studyof pull in and pull out processes associated with electrical propertiesof NEMS structures.

Other description and examples can be found in: M. S. Kabir, “Towardsthe Integration of Carbon Nanostructures into CMOS Technology”, Ph.D.Thesis, Chalmers University of Technology, Göteborg, Sweden, (August2005), ISBN: 91-7291-648-6, incorporated herein by reference.

The foregoing description is intended to illustrate various aspects ofthe present invention. It is not intended that the examples presentedherein limit the scope of the present invention. The invention now beingfully described, it will be apparent to one of ordinary skill in the artthat many changes and modifications can be made thereto withoutdeparting from the spirit or scope of the appended claims.

All references cited herein are hereby incorporated by reference intheir entirety for all purposes.

1. A nanostructure assembly comprising: a conducting substrate; ananostructure supported by the conducting substrate; and a plurality ofintermediate layers between the conducting substrate and thenanostructure, wherein at least two of the plurality of intermediatelayers are interdiffused, and wherein material of the at least two ofthe plurality of intermediate layers that are interdiffused are presentin the nanostructure, the plurality of intermediate layers including atleast one layer that affects a morphology of the nanostructure and atleast one layer to affect an electrical property of an interface betweenthe conducting substrate and the nanostructure, and wherein at least oneof the plurality of intermediate layers comprises material selected fromthe group consisting of amorphous silicon and germanium.
 2. Thenanostructure of claim 1 wherein the conducting substrate comprises ametal.
 3. The nanostructure of claim 2 wherein the metal is selectedfrom the group consisting of tungsten, molybdenum, niobium, platinum andpalladium.
 4. The nanostructure assembly of claim 1 wherein theplurality of intermediate layers comprises a metal layer and a layer ofsemiconducting material.
 5. The nanostructure of claim 4 wherein thelayer of semiconducting material is amorphous silicon.
 6. Thenanostructure assembly of claim 1, wherein the nanostructure is a carbonnanotube.
 7. The nanostructure assembly of claim 1, wherein thenanostructure is a carbon nanofiber.
 8. The nanostructure assembly ofclaim 7, wherein the nanostructure has a conical angle less than 2°. 9.The nanostructure assembly of claim 1, wherein the nanostructure is madefrom a compound selected from the group consisting of: InP, GaAs, andAlGaAs.
 10. The nanostructure assembly of claim 1, wherein the pluralityof intermediate layers form an Ohmic contact.
 11. The nanostructureassembly of claim 1, wherein the plurality of intermediate layers form aSchottky barrier.
 12. An array of nanostructure assemblies according toclaim 1, wherein the conducting substrate is directly on a wafer ofsilicon, or oxidized silicon.
 13. The nanostructure assembly of claim 1,wherein the plurality of intermediate layers is between 1 nm and 1 μmthick.
 14. The nanostructure assembly of claim 1, wherein theintermediate layer adjacent to the nanostructure is a layer of catalyst,and wherein the catalyst is selected from the group consisting of Ni,Fe, Mo, NiCr, and Pd.
 15. A carbon nanostructure assembly comprising: aconducting substrate; a layer of amorphous silicon on the conductingsubstrate; and a layer of catalyst on the layer of amorphous silicon,wherein the carbon nanostructure is disposed on the catalyst, andwherein the layer of catalyst and the layer of amorphous silicon areinterdiffused, and wherein material of the layer of catalyst and thelayer of amorphous silicon that are interdiffused are present in thecarbon nanostructure.
 16. A method of forming a nanostructure,comprising: depositing a layer of semiconducting material on aconducting substrate; depositing one or more intermediate layers,wherein at least one of the plurality of intermediate layers comprisesmaterial selected from the group consisting of amorphous silicon andgermanium; depositing a catalyst layer on the one or more intermediatelayers; without first annealing the substrate, causing the substrate tobe heated to a temperature at which the nanostructure can form; andgrowing a nanostructure on the catalyst layer at the temperature,wherein at least one of the one or more intermediate layers areinterdiffused with the catalyst layer, and wherein material of thecatalyst layer and the at least one of the one or more intermediatelayers that are interdiffused are present in the nanostructure. 17.(canceled)
 18. (canceled)
 19. (canceled)
 20. (canceled)
 21. Thenanostructure assembly of claim 1, additionally comprising a catalystlayer adjacent to the nanostructure, wherein the plurality ofintermediate layers are not the same material as the catalyst.
 22. Thenanostructure assembly of claim 21, wherein the catalyst layer issupporting or supported by the nanostructure, and the plurality ofintermediate layers lies between the conducting substrate and whicheverof the nanostructure and the catalyst layer is closer to the conductingsubstrate.
 23. The nanostructure assembly of claim 21, wherein thecatalyst in the catalyst layer is selected from Ni, Fe, Mo, NiCr, andPd.
 24. The nanostructure assembly of claim 21, wherein the catalystlayer is supporting the nanostructure, and further comprising a secondcatalyst layer supported by the nanostructure.
 25. An array of carbonnanostructures supported on a substrate, wherein each carbonnanostructure in the array comprises a nanostructure assembly accordingto claim 1, and wherein said each carbon nanostructure is spaced apartfrom any other carbon nanostructure in the array by between 15 nm and200 nm.
 26. The method of claim 16, wherein the method further comprisesdepositing a metal layer on the layer of semiconducting material, andwherein the catalyst layer is deposited on the metal layer, and wherein,during the growing, the metal layer interdiffuses with the catalystlayer, and wherein material of the metal layer and the catalyst layerthat are interdiffused become present in the nanostructure.
 27. Thenanostructure assembly of claim 5, wherein the layer of amorphoussilicon, the layer of catalyst, and at least one of the plurality ofintermediate layers have interdiffused with each other, and whereinmaterial of the layer of amorphous silicon, the layer of catalyst, andthe at least one of the plurality of intermediate layers that areinterdiffused are present in the nanostructure.
 28. The nanostructureassembly of claim 27, wherein the nanostructure has a tip and a base,and wherein material of the layer of amorphous silicon, the layer ofcatalyst, and the at least one of the plurality of intermediate layersthat are interdiffused are present both at the tip and at the base ofthe nanostructure.
 29. The nanostructure assembly of claim 14, whereinthe layer of catalyst has interdiffused with at least one of theplurality of intermediate layers, and wherein material of the layer ofcatalyst and the at least one of the plurality of intermediate layersthat are interdiffused are present in the nanostructure.
 30. Thenanostructure assembly of claim 29, wherein the nanostructure has a tipand a base, and wherein material of the layer of catalyst and the atleast one of the plurality of intermediate layers that are interdiffusedare present both at the tip and at the base of the nanostructure. 31.The nanostructure assembly of claim 4, wherein at least one of theplurality of intermediate layers has interdiffused into the metal layerand into the semiconducting layer, and wherein material of the catalystlayer, the metal layer, and the semiconducting layer that areinterdiffused are present in the nanostructure.
 32. The nanostructureassembly of claim 31, wherein the nanostructure has a tip and a base,and wherein material of the layer of catalyst, the metal layer, and thesemiconducting layer that are interdiffused are present both at the tipand at the base of the nanostructure.
 33. The nanostructure assembly ofclaim 1 wherein the plurality of intermediate layers comprises: a firstcontrol layer disposed on the conducting substrate; a metal/semi-metallayer disposed on the first control layer, wherein the first controllayer controls diffusion from the metal/semi-metal layer into theconducting substrate; a second control layer disposed on themetal/semi-metal layer; and one or more layers disposed on the secondcontrol layer.
 34. The nanostructure assembly of claim 33 wherein thefirst control layer comprises material selected from the groupconsisting of Ta, Ti, TiN, SiN_(x), SiO_(x), and Al₂O₃.
 35. Thenanostructure assembly of claim 33 wherein the second control layercomprises high k dielectric material.
 36. The nanostructure assembly ofclaim 35 wherein the second control layer comprises material selectedfrom the group consisting of ZrO_(x), HfO_(x), SiN_(x), Ta₂O₅, Al₂O₃,and TiO₂.